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()