Give street props colliders, zoned placements and variants

Props were decoration: the guard and guests walked straight through benches,
bins, bollards and planters. They now block movement and steer guests around.

Collision
- _add_prop_model() fits a StaticBody3D from the model's mesh bounds in
  holder-local space: a cylinder for bollard/bin/hydrant, a box otherwise, so
  placement rotation and variant scale carry over.
- Fallback primitives go through the same fit, so a missing .glb cannot silently
  remove collision.
- Each prop also publishes a NavigationObstacle3D; NpcAvatar enables agent
  avoidance and moves with the safe velocity from velocity_computed. The navmesh
  stays a flat walkable plane and avoidance does the steering.

Placement
- PROP_PLACEMENTS groups furniture by entrance, mid-block and curb zones, with a
  static placement_problems() validator for bounds, overlap/min-spacing,
  duplicates, and the door/steps/guest-lane exclusions.
- variant_for(index) gives deterministic yaw jitter, subtle scale, and optional
  clutter (bag by the bin, crate by the newspaper boxes, cone by the hydrant).

Verification
- New windowed smoke_props.gd: guard walks into a bench and stops; a guest
  headed beyond the bench routes around it and never enters the collider.
- Headless 182 -> 187: placement validation and its failure modes, variant
  determinism/range, collider + obstacle on every placed prop, fallback collider.
- Other smokes (PLAN verbs, crowd, emergency, timeline) still pass; Python
  tooling 53/53; screenshots recaptured; compare 9 good / 0 bad.
- Version 0.4.0. TODO: collision/placement section moved to DONE; prop pipeline
  checks remain.

src/game/npc_avatar.gd

@@ -73,6 +73,15 @@ func setup(profile: NpcProfile, intent: String, seed_value: int, p_spawn: Vector
 	agent.target_desired_distance = 4.0
 	agent.radius = 0.35
 	agent.height = 1.2
+	# Steer around street furniture instead of walking into it (props publish a
+	# NavigationObstacle3D; the navmesh itself stays a flat walkable plane).
+	agent.avoidance_enabled = true
+	agent.neighbor_distance = 4.0
+	agent.max_neighbors = 8
+	agent.time_horizon_agents = 0.6
+	agent.time_horizon_obstacles = 1.2
+	agent.max_speed = FLEE_SPEED
+	agent.velocity_computed.connect(_on_safe_velocity)
 	add_child(agent)
 
 	# Normalize intent targets: approach the entrance, stop near the door.
@@ -98,6 +107,7 @@ var _sign_prop: MeshInstance3D = null
 var _legs: Array[MeshInstance3D] = []
 var _arms: Array[MeshInstance3D] = []
 var _theater_t := 0.0
+var _avoidance_active := false
 var _photo_prop_base_y := 0.0
 var _sign_prop_base_y := 0.0
 const SKIN_TONE := Color(0.88, 0.72, 0.60)
@@ -527,6 +537,7 @@ func _advance_fsm(delta: float) -> void:
 var leave_requested_by_lifetime := false
 
 func _move_toward_target(delta: float) -> void:
+	_avoidance_active = false
 	if agent == null:
 		_move_direct(delta)
 		return
@@ -575,14 +586,33 @@ func _move_toward_target(delta: float) -> void:
 		return
 	dir = dir.normalized()
 	var speed := FLEE_SPEED if reaction_mode == "flee" else 1.3
-	velocity.x = dir.x * speed
-	velocity.z = dir.z * speed
-	velocity.y -= 9.8 * delta
 	look_at(global_position + Vector3(dir.x, 0, dir.z), Vector3.UP)
+	if agent.avoidance_enabled:
+		# The navigation server returns the obstacle-aware velocity through
+		# velocity_computed; never dash straight at the point in the meantime.
+		_avoidance_active = true
+		agent.velocity = Vector3(dir.x * speed, 0.0, dir.z * speed)
+	else:
+		velocity.x = dir.x * speed
+		velocity.z = dir.z * speed
+		velocity.y -= 9.8 * delta
+		move_and_slide()
+
+## Safe velocity from NavigationServer avoidance: move with the steered vector.
+func _on_safe_velocity(safe: Vector3) -> void:
+	if not _avoidance_active:
+		return
+	velocity.x = safe.x
+	velocity.z = safe.z
+	if is_on_floor():
+		velocity.y = 0.0
+	else:
+		velocity.y -= 9.8 * get_physics_process_delta_time()
 	move_and_slide()
 
 ## Fallback used when no navigation mesh is available (e.g. minimal scenes).
 func _move_direct(delta: float) -> void:
+	_avoidance_active = false
 	var to_target := target_position - global_position
 	to_target.y = 0.0
 	var dist := to_target.length()

src/game/world_builder.gd

@@ -335,35 +335,112 @@ func _build_street_furniture() -> void:
 		tree.add_child(crown)
 		root.add_child(tree)
 
+## Street-furniture placement table. Positions are in world space on the
+## sidewalk top (y = 0.18) and grouped by zone, so a cluster reads as street
+## furniture instead of a scattered list. `accessory` names a small procedural
+## addition (bag, crate, cone) for one-off variation.
+const PROP_PLACEMENTS := [
+	# Entrance: bollards flank the foot of the steps, bins catch people who
+	# stop at the door.
+	{"zone": "entrance", "prop": "bollard", "x": -5.6, "z": 4.7},
+	{"zone": "entrance", "prop": "bollard", "x": -4.2, "z": 4.7},
+	{"zone": "entrance", "prop": "bollard", "x": 4.2, "z": 4.7},
+	{"zone": "entrance", "prop": "bollard", "x": 5.6, "z": 4.7},
+	{"zone": "entrance", "prop": "trash_bin", "x": -6.6, "z": 4.9, "accessory": "bag"},
+	{"zone": "entrance", "prop": "trash_bin", "x": 6.6, "z": 4.9},
+	# Mid-block: benches face the boulevard, planters break up the long run.
+	{"zone": "mid_block", "prop": "bench", "x": -8.8, "z": 6.4, "rot": 180.0},
+	{"zone": "mid_block", "prop": "bench", "x": 8.8, "z": 6.4, "rot": 180.0},
+	{"zone": "mid_block", "prop": "planter_box", "x": -11.8, "z": 8.8},
+	{"zone": "mid_block", "prop": "planter_box", "x": 11.8, "z": 8.8},
+	# Curb: newspaper boxes and a hydrant along the roadway edge.
+	{"zone": "curb", "prop": "newspaper_box", "x": -9.6, "z": 11.0, "rot": -12.0},
+	{"zone": "curb", "prop": "newspaper_box", "x": -8.9, "z": 11.0, "rot": -12.0,
+		"accessory": "crate"},
+	{"zone": "curb", "prop": "fire_hydrant", "x": 8.4, "z": 11.2,
+		"accessory": "cone"},
+]
+
+## The door, steps, and the lane guests walk in through stay clear of props.
+const PROP_EXCLUSIONS := [
+	{"name": "steps and door", "x0": -4.6, "x1": 4.6, "z0": -4.6, "z1": 4.55},
+	{"name": "guest approach lane", "x0": -1.4, "x1": 1.4, "z0": 4.55, "z1": 14.0},
+]
+const PROP_BOUNDS := {"x0": -14.0, "x1": 14.0, "z0": 4.55, "z1": 11.8}
+## Two prop centres closer than this read as one blob (newspaper boxes are
+## deliberately lined up, so this stays below their 0.7 m spacing).
+const PROP_MIN_SPACING := 0.6
+
+static func _variant_seed(index: int) -> int:
+	return index * 7919 + 13
+
+## Deterministic per-placement variation. The same index always gives the same
+## rotation and scale, so the street is reproducible while repeats differ.
+static func variant_for(index: int, base_rotation: float = 0.0) -> Dictionary:
+	var rng := RandomNumberGenerator.new()
+	rng.seed = _variant_seed(index)
+	return {
+		"rotation": base_rotation + rng.randf_range(-2.0, 2.0),
+		"scale": rng.randf_range(0.96, 1.04),
+		"roll": rng.randf(),
+	}
+
+## Static validation used by tests: reject duplicate, out-of-bounds, or
+## overlapping placements, and any prop inside the door/steps/approach lane.
+static func placement_problems(placements: Array = PROP_PLACEMENTS) -> Array[String]:
+	var problems: Array[String] = []
+	var seen := {}
+	for i in placements.size():
+		var entry: Dictionary = placements[i]
+		var prop := str(entry.get("prop", ""))
+		var x := float(entry.get("x", 0.0))
+		var z := float(entry.get("z", 0.0))
+		var key := "%s@%.2f,%.2f" % [prop, x, z]
+		if seen.has(key):
+			problems.append("duplicate placement %s" % key)
+		seen[key] = true
+		if x < float(PROP_BOUNDS["x0"]) or x > float(PROP_BOUNDS["x1"]) \
+				or z < float(PROP_BOUNDS["z0"]) or z > float(PROP_BOUNDS["z1"]):
+			problems.append("%s at (%.1f, %.1f) is outside the sidewalk" % [prop, x, z])
+		for zone in PROP_EXCLUSIONS:
+			if x > float(zone["x0"]) and x < float(zone["x1"]) \
+					and z > float(zone["z0"]) and z < float(zone["z1"]):
+				problems.append("%s at (%.1f, %.1f) blocks %s" % [prop, x, z, zone["name"]])
+		for j in range(i):
+			var other: Dictionary = placements[j]
+			var dx := x - float(other.get("x", 0.0))
+			var dz := z - float(other.get("z", 0.0))
+			if Vector2(dx, dz).length() < PROP_MIN_SPACING:
+				problems.append("%s and %s at (%.1f, %.1f) overlap" % [
+					prop, str(other.get("prop", "")), x, z])
+	return problems
+
 ## Street furniture built from the generated GLB props (tools/model_pipeline.py).
 ## The meshes are real 3D models (bollards, benches, bins, planters, newspaper
-## boxes, a hydrant) rather than scaled boxes, and every placement is fixed so
-## the street looks the same on every run.
+## boxes, a hydrant) rather than scaled boxes. Placements and their deterministic
+## variants are fixed, so the street looks the same on every run.
 func _build_street_detail() -> void:
 	var detail := Node3D.new()
 	detail.name = "StreetDetail"
 	root.add_child(detail)
-	# Bollards flank the foot of the steps.
-	for x in [-5.6, -4.2, 4.2, 5.6]:
-		_add_prop_model(detail, "bollard", Vector3(x, 0.18, 4.7))
-	# Benches face the boulevard.
-	_add_prop_model(detail, "bench", Vector3(-8.8, 0.18, 6.4), 180.0)
-	_add_prop_model(detail, "bench", Vector3(8.8, 0.18, 6.4), 180.0)
-	# Bins where people leave the steps.
-	_add_prop_model(detail, "trash_bin", Vector3(-6.6, 0.18, 4.9))
-	_add_prop_model(detail, "trash_bin", Vector3(6.6, 0.18, 4.9))
-	# Planters break up the long sidewalk run.
-	_add_prop_model(detail, "planter_box", Vector3(-11.8, 0.18, 8.8))
-	_add_prop_model(detail, "planter_box", Vector3(11.8, 0.18, 8.8))
-	# Newspaper boxes and a hydrant along the curb.
-	_add_prop_model(detail, "newspaper_box", Vector3(-9.6, 0.18, 11.0), -12.0)
-	_add_prop_model(detail, "newspaper_box", Vector3(-8.9, 0.18, 11.0), -12.0)
-	_add_prop_model(detail, "fire_hydrant", Vector3(8.4, 0.18, 11.2))
+	for index in PROP_PLACEMENTS.size():
+		var entry: Dictionary = PROP_PLACEMENTS[index]
+		var base_rot := float(entry.get("rot", 0.0))
+		var variant := variant_for(index, base_rot)
+		var holder := _add_prop_model(detail, str(entry["prop"]),
+			Vector3(float(entry["x"]), 0.18, float(entry["z"])),
+			float(variant["rotation"]), index)
+		holder.scale = Vector3.ONE * float(variant["scale"])
+		var accessory := str(entry.get("accessory", ""))
+		if accessory != "":
+			_add_prop_accessory(holder, accessory)
 
 ## Place one generated prop. Falls back to simple primitives when the `.glb` is
 ## missing (fresh checkout before `godot --import`) so the world still builds.
+## A static collider is fitted from the model bounds (cylinder for round props,
+## box for everything else), so props block the guard and NPCs.
 func _add_prop_model(parent: Node3D, prop: String, pos: Vector3,
-		rot_y_deg: float = 0.0) -> Node3D:
+		rot_y_deg: float = 0.0, variant_index: int = -1) -> Node3D:
 	var holder := Node3D.new()
 	holder.name = "Prop_%s_%d" % [prop, parent.get_child_count()]
 	holder.position = pos
@@ -373,11 +450,97 @@ func _add_prop_model(parent: Node3D, prop: String, pos: Vector3,
 		holder.add_child(model)
 		loaded_models.append(prop)
 	else:
-		holder.add_child(_fallback_prop(prop))
+		var fallback := _fallback_prop(prop)
+		holder.add_child(fallback)
 		fallback_props.append(prop)
+		model = fallback
 	parent.add_child(holder)
+	_add_prop_collision(holder, prop, model)
 	return holder
 
+## Fit a simple collider to the prop's meshes (in holder-local space, so the
+## placement scale and rotation carry over).
+func _add_prop_collision(holder: Node3D, prop: String, model: Node3D) -> void:
+	var bounds := _model_bounds(model, holder)
+	if bounds.size == Vector3.ZERO:
+		return
+	var body := StaticBody3D.new()
+	body.name = "Collision"
+	var collider := CollisionShape3D.new()
+	collider.name = "Shape"
+	var round_props := ["bollard", "trash_bin", "fire_hydrant"]
+	if prop in round_props:
+		var cylinder := CylinderShape3D.new()
+		cylinder.radius = maxf(bounds.size.x, bounds.size.z) * 0.5
+		cylinder.height = bounds.size.y
+		collider.shape = cylinder
+	else:
+		var box := BoxShape3D.new()
+		box.size = bounds.size
+		collider.shape = box
+	collider.position = bounds.get_center()
+	body.add_child(collider)
+	holder.add_child(body)
+	# Publish the footprint to NavigationServer so guest avoidance steers around
+	# the prop even though the navmesh itself is a flat walkable plane.
+	var obstacle := NavigationObstacle3D.new()
+	obstacle.name = "Avoidance"
+	obstacle.radius = maxf(bounds.size.x, bounds.size.z) * 0.5 + 0.25
+	obstacle.height = maxf(bounds.size.y, 0.5)
+	obstacle.position = Vector3(bounds.get_center().x, 0.0, bounds.get_center().z)
+	holder.add_child(obstacle)
+
+## AABB of every mesh under `model`, expressed relative to `ancestor`.
+func _model_bounds(model: Node3D, ancestor: Node3D) -> AABB:
+	var bounds := AABB()
+	var found := false
+	for node in model.find_children("*", "MeshInstance3D", true, false):
+		var mi := node as MeshInstance3D
+		if mi.mesh == null:
+			continue
+		var xform := _transform_to_ancestor(mi, ancestor)
+		var box: AABB = xform * mi.mesh.get_aabb()
+		bounds = box if not found else bounds.merge(box)
+		found = true
+	return bounds
+
+func _transform_to_ancestor(node: Node, ancestor: Node) -> Transform3D:
+	var xform := Transform3D.IDENTITY
+	var current := node
+	while current != null and current != ancestor:
+		if current is Node3D:
+			xform = (current as Node3D).transform * xform
+		current = current.get_parent()
+	return xform
+
+## Small deterministic clutter next to a prop: a bag by the bin, a crate by the
+## newspaper boxes, a cone by the hydrant.
+func _add_prop_accessory(holder: Node3D, kind: String) -> void:
+	match kind:
+		"bag":
+			var bag := MeshInstance3D.new()
+			var sphere := SphereMesh.new()
+			sphere.radius = 0.22
+			sphere.height = 0.5
+			bag.mesh = sphere
+			bag.material_override = Materials.flat(Color(0.10, 0.11, 0.13), "rubber")
+			bag.position = Vector3(0.38, 0.22, 0.06)
+			holder.add_child(bag)
+			_add_box(holder, Vector3(0.12, 0.10, 0.12), Vector3(0.38, 0.48, 0.06),
+				Color(0.16, 0.17, 0.20), false, null, Vector2.ONE, "rubber")
+		"crate":
+			_add_box(holder, Vector3(0.46, 0.40, 0.40), Vector3(0.48, 0.20, 0.08),
+				Color(0.45, 0.31, 0.18), true, null, Vector2.ONE, "wood")
+			_add_box(holder, Vector3(0.48, 0.03, 0.42), Vector3(0.48, 0.41, 0.08),
+				Color(0.35, 0.23, 0.13), false, null, Vector2.ONE, "wood")
+		"cone":
+			_add_cylinder_mesh(holder, 0.16, 0.55, Vector3(-0.42, 0.28, 0.10),
+				Color(0.95, 0.36, 0.10), "painted_metal")
+			_add_box(holder, Vector3(0.38, 0.05, 0.38), Vector3(-0.42, 0.03, 0.10),
+				Color(0.12, 0.12, 0.14), false, null, Vector2.ONE, "rubber")
+			_add_cylinder_mesh(holder, 0.11, 0.08, Vector3(-0.42, 0.34, 0.10),
+				Color(0.92, 0.92, 0.9), "painted_metal")
+
 func _load_prop_scene(prop: String) -> Node3D:
 	var path := MODEL_DIR + prop + ".glb"
 	if not ResourceLoader.exists(path):

src/test/smoke_props.gd

@@ -0,0 +1,165 @@
+extends SceneTree
+## Prop-collision smoke: proves the guard and an NPC cannot walk through street
+## furniture, and that a guest slides around a bench instead of passing through
+## it. Windowed (the guard reads captured-mouse input):
+##
+##   godot --path . --script res://src/test/smoke_props.gd
+##
+## Exit code 0 = pass, 1 = failure.
+
+const MAX_FRAMES := 1800
+const GUARD_PUSH_FRAMES := 100
+
+var frame := 0
+var gm: Node = null
+var started := false
+var step := 0
+var step_frame := 0
+var failures: Array[String] = []
+var bench: Node3D = null
+var bench_center := Vector3.ZERO
+var bench_body: StaticBody3D = null
+var guard_start := Vector3.ZERO
+var npc: Node3D = null
+var npc_start := Vector3.ZERO
+
+func _initialize() -> void:
+	root.size = Vector2i(1280, 720)
+	root.add_child(load("res://scenes/main.tscn").instantiate())
+
+func _process(_delta: float) -> bool:
+	frame += 1
+	if gm == null:
+		gm = root.find_child("GameManager", true, false)
+		return false
+	if not started:
+		gm.call("_on_venue_selected", "hollywood_building")
+		Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
+		started = true
+		return false
+	if frame > MAX_FRAMES:
+		_fail("timeout before the collision checks finished")
+		return true
+	step_frame += 1
+	match step:
+		0:
+			_find_bench()
+			if bench == null:
+				return false
+			# Stand a couple of metres in front of the bench and walk into it.
+			var start := bench_center + Vector3(0, 0, 2.2)
+			start.y = 0.1
+			gm.guard.global_position = start
+			# Face the bench (-z): with the guard's basis, forward input at yaw PI
+			# walks toward lower z.
+			gm.guard.rotation.y = PI
+			guard_start = start
+			Input.action_press("move_forward")
+			step = 1
+			step_frame = 0
+		1:
+			if step_frame >= GUARD_PUSH_FRAMES:
+				Input.action_release("move_forward")
+				_check(guard_start.z - gm.guard.global_position.z > 0.5,
+					"guard walked toward the bench")
+				_check(gm.guard.global_position.z > bench_center.z + 0.25,
+					"guard stopped at the bench instead of passing through it")
+				_check(not _inside_bench(gm.guard.global_position),
+					"guard is not inside the bench collider")
+				# Step the guard aside so the guest test hits the bench, not the player.
+				gm.guard.global_position = bench_center + Vector3(4.0, -0.08, 2.2)
+				_stage_npc()
+				step = 2
+				step_frame = 0
+		2:
+			if step_frame % 120 == 0 and npc != null and is_instance_valid(npc):
+				print("npc dbg pos=%s vel=%s arrived=%s wait=%s" % [str(npc.global_position), str(npc.velocity), str(npc.arrived), str(npc.awaiting_interaction)])
+			if npc == null or not is_instance_valid(npc):
+				_stage_npc()
+				return false
+			if _npc_finished():
+				_check(npc.global_position.z < bench_center.z - 0.4,
+					"guest reached the far side of the bench")
+				_check(not _inside_bench(npc.global_position),
+					"guest is not inside the bench collider")
+				_check(npc_start.distance_to(npc.global_position) > 1.0,
+					"guest moved while routing around the prop")
+				_finish()
+				return true
+	return false
+
+func _find_bench() -> void:
+	var detail := gm.get_node_or_null("World/StreetDetail")
+	if detail == null:
+		# The world root name differs between scene builds; search instead.
+		detail = root.find_child("StreetDetail", true, false)
+	if detail == null:
+		_fail("StreetDetail not found")
+		return
+	for child in detail.get_children():
+		if str(child.name).begins_with("Prop_bench"):
+			bench = child as Node3D
+			break
+	if bench == null:
+		_fail("no bench prop placed")
+		return
+	bench_center = bench.global_position
+	bench_body = bench.get_node_or_null("Collision") as StaticBody3D
+	if bench_body == null:
+		_fail("bench has no static collider")
+
+func _stage_npc() -> void:
+	if gm.npcs.is_empty():
+		step = 0
+		step_frame = 0
+		return
+	npc = gm.npcs[0]
+	npc_start = bench_center + Vector3(0, 0.5, 2.5)
+	npc.global_position = npc_start
+	npc.velocity = Vector3.ZERO
+	npc.arrived = false
+	npc.awaiting_interaction = false
+	npc.lifetime_s = 999.0
+	npc.reaction_mode = ""
+	# Target well past the bench forces a straight line through it, and stays
+	# beyond the controller's 3 m "arrived" shortcut so the guest must route.
+	npc.target_position = Vector3(bench_center.x, 0.5, bench_center.z - 5.0)
+
+func _npc_finished() -> bool:
+	if npc == null or not is_instance_valid(npc):
+		return false
+	return npc.global_position.z < bench_center.z - 0.4 \
+		or npc.global_position.distance_to(npc.target_position) < 0.9
+
+func _inside_bench(point: Vector3) -> bool:
+	if bench_body == null:
+		return false
+	var collider := bench_body.get_node_or_null("Shape") as CollisionShape3D
+	if collider == null or not (collider.shape is BoxShape3D):
+		return false
+	var box := collider.shape as BoxShape3D
+	var half := box.size * 0.5
+	var center := collider.global_position
+	half.x += 0.1
+	half.z += 0.1
+	return absf(point.x - center.x) < half.x and absf(point.z - center.z) < half.z
+
+func _check(condition: bool, message: String) -> void:
+	if condition:
+		print("ok: " + message)
+	else:
+		_fail(message)
+
+func _fail(message: String) -> void:
+	failures.append(message)
+	_finish()
+
+func _finish() -> void:
+	Input.action_release("move_forward")
+	if failures.is_empty():
+		print("SMOKE OK: props block the guard and guests route around them")
+		quit(0)
+	else:
+		for message in failures:
+			printerr("SMOKE FAIL: " + message)
+		quit(1)

src/test/tests/test_generated_props.gd

@@ -83,6 +83,96 @@ func test_world_places_generated_props() -> bool:
 	root.free()
 	return a and b and c and d
 
+func test_prop_placements_are_valid() -> bool:
+	var problems := WorldBuilder.placement_problems()
+	return TestUtils.check(problems.is_empty(),
+		"prop placements keep the door, steps and approach lane clear (%s)" % str(problems))
+
+func test_placement_validator_catches_problems() -> bool:
+	var bad: Array = WorldBuilder.PROP_PLACEMENTS.duplicate(true)
+	bad.append({"zone": "test", "prop": "bench", "x": 0.0, "z": 2.0})
+	bad.append({"zone": "test", "prop": "bench", "x": 0.0, "z": 2.0})
+	var problems := WorldBuilder.placement_problems(bad)
+	var a := TestUtils.check(not problems.is_empty(), "validator rejects bad placements")
+	var joined := " ".join(problems)
+	var b := TestUtils.check(joined.contains("steps and door"),
+		"validator reports the door/steps exclusion")
+	var c := TestUtils.check(joined.contains("duplicate"), "validator reports a duplicate")
+	return a and b and c
+
+func test_prop_variants_are_deterministic() -> bool:
+	var first := WorldBuilder.variant_for(3, 180.0)
+	var second := WorldBuilder.variant_for(3, 180.0)
+	var other := WorldBuilder.variant_for(4, 180.0)
+	var a := TestUtils.check(is_equal_approx(first["rotation"], second["rotation"])
+		and is_equal_approx(first["scale"], second["scale"]),
+		"the same placement always gets the same variant")
+	var b := TestUtils.check(not is_equal_approx(first["rotation"], other["rotation"]),
+		"different placements get different rotations")
+	var c := TestUtils.check(first["scale"] >= 0.95 and first["scale"] <= 1.05,
+		"variant scale stays subtle")
+	var d := TestUtils.check(absf(first["rotation"] - 180.0) <= 2.01,
+		"variant rotation stays near the authored angle")
+	return a and b and c and d
+
+func test_every_prop_has_a_collider() -> bool:
+	var root := Node3D.new()
+	var builder := WorldBuilder.new(root)
+	builder.build()
+	var detail := root.get_node_or_null("StreetDetail")
+	var problems: Array[String] = []
+	if detail == null:
+		problems.append("no StreetDetail")
+	else:
+		for holder in detail.get_children():
+			var body := holder.get_node_or_null("Collision")
+			if body == null or not (body is StaticBody3D):
+				problems.append("%s has no StaticBody3D" % holder.name)
+				continue
+			var collider := body.get_node_or_null("Shape") as CollisionShape3D
+			if collider == null or collider.shape == null:
+				problems.append("%s has no collision shape" % holder.name)
+				continue
+			var size: Vector3
+			if collider.shape is BoxShape3D:
+				size = (collider.shape as BoxShape3D).size
+			elif collider.shape is CylinderShape3D:
+				var cylinder := collider.shape as CylinderShape3D
+				size = Vector3(cylinder.radius * 2.0, cylinder.height, cylinder.radius * 2.0)
+			else:
+				problems.append("%s uses an unexpected shape" % holder.name)
+				continue
+			if size.x <= 0.05 or size.y <= 0.05 or size.z <= 0.05:
+				problems.append("%s has a degenerate collider" % holder.name)
+			if collider.position.y + size.y * 0.5 <= 0.05:
+				problems.append("%s collider sits below the ground" % holder.name)
+			if holder.get_node_or_null("Avoidance") == null:
+				problems.append("%s has no navigation obstacle" % holder.name)
+	root.free()
+	return TestUtils.check(problems.is_empty(), "every prop has a static collider (%s)" % str(problems))
+
+func test_fallback_props_get_the_same_collider() -> bool:
+	## A missing `.glb` must not silently remove collision: fallback primitives go
+	## through the same bounds fit as the imported models.
+	var root := Node3D.new()
+	var builder := WorldBuilder.new(root)
+	var holder := Node3D.new()
+	var fallback := builder._fallback_prop("bench")
+	holder.add_child(fallback)
+	builder._add_prop_collision(holder, "bench", fallback)
+	var body := holder.get_node_or_null("Collision") as StaticBody3D
+	var a := TestUtils.check(body != null, "fallback bench gets a static body")
+	var collider := body.get_node_or_null("Shape") as CollisionShape3D if body else null
+	var b := TestUtils.check(collider != null and collider.shape is BoxShape3D,
+		"fallback bench gets a box shape")
+	var width: float = (collider.shape as BoxShape3D).size.x if b else 0.0
+	var c := TestUtils.check(width > 1.0, "fallback collider follows the bench width (%.2f)" % width)
+	var d := TestUtils.check(holder.get_node_or_null("Avoidance") is NavigationObstacle3D,
+		"fallback bench publishes a navigation obstacle")
+	holder.free()
+	root.free()
+	return a and b and c and d
+
 func test_world_facade_windows_light_at_night() -> bool:
 	var root := Node3D.new()
 	var builder := WorldBuilder.new(root)