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.

DONE.md

@@ -709,3 +709,28 @@ Python tooling **53/53**, headless **182/182**.
       the reference matches the game camera.
 - [x] Added `Hud.clear_log()` so staged captures drop target-cycling noise; the escalation shot
       keeps the real reaction lines.
+
+## Prop Collision And Placement
+
+Completed 2026-11-01 (v0.4.0). Headless **187/187**, Python tooling **53/53**, the new
+`smoke_props.gd` passes windowed, and `screen_designs.py compare` still reports 9 good / 0 bad.
+
+- [x] Every generated prop now gets a fitted static collider: a cylinder for bollard, bin, and
+      hydrant, a box for everything else. The collider is computed from the model's mesh bounds in
+      holder-local space, so placement rotation and variant scale carry over.
+- [x] Added `smoke_props.gd` (windowed): the guard walks into a bench and stops instead of passing
+      through it; a guest heading for a point beyond the same bench routes around it with
+      navigation avoidance and never ends up inside the collider.
+- [x] Props publish a `NavigationObstacle3D` footprint, and `NpcAvatar` enables agent avoidance with
+      a `velocity_computed` callback. The navmesh stays a flat walkable plane; avoidance does the
+      steering around street furniture.
+- [x] Replaced the hard-coded prop calls with `WorldBuilder.PROP_PLACEMENTS`, grouped 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.
+- [x] Deterministic variants per placement (`variant_for`): yaw jitter, subtle scale, and optional
+      procedural clutter (bag by the bin, crate by the newspaper boxes, cone by the hydrant), so
+      repeated props stop looking cloned.
+- [x] Fallback primitive props go through the same bounds fit, so a missing `.glb` cannot silently
+      remove collision; a unit test covers the fallback body, shape, width, and navigation obstacle.
+- [x] Tests: placement validation and its failure modes, variant determinism/range, colliders on
+      every placed prop (including obstacle presence), and the fallback collider path.