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.

MEMORY.md

@@ -46,11 +46,15 @@ Session memory for durable decisions, state, and gotchas. Keep concise, remove o
   until it alerts, then jump to 760 s after staging for night lighting. The timeline needs
   `recorder.close_all_open()`, because `to_dict()` only lists closed incidents. The escalation
   HUD mockup is third-person now; `judge` still classes it `review` (no diff).
-- **Presentation pass planned (TODO):** three sections — props, NPC characters, player movement.
-  Grounding facts: generated props currently have **no colliders** (only ground/building/steps do),
-  prop placement is a fixed list in `_build_street_detail()`, the NPC rig is rigid capsules with a
-  whole-limb swing, and `guard_controller.gd` uses one `accel` value. Work through the sections in
-  order; each task should get its own windowed smoke or capture where it changes feel/looks.
+- **Prop collision (v0.4.0):** `_add_prop_model()` fits a `StaticBody3D` from the model bounds
+  (cylinder for bollard/bin/hydrant, box otherwise) and adds a `NavigationObstacle3D`; NPC agents
+  enable avoidance and move with the safe velocity from `velocity_computed`. The navmesh stays a
+  flat plane, so avoidance (not a baked mesh) routes guests around props. Placements live in
+  `PROP_PLACEMENTS` with `placement_problems()` validation; variants come from `variant_for(index)`.
+  `smoke_props.gd` must be windowed because the guard reads captured-mouse input.
+- **Presentation pass remaining (TODO):** NPC characters and player movement. Grounding facts: the
+  NPC rig is rigid capsules with a whole-limb swing, and `guard_controller.gd` uses one `accel`
+  value. Each task should get its own windowed smoke or capture where it changes feel/looks.
 - **Backlog complete:** every planned TODO section is done and moved to `DONE.md`; `TODO.md`
   now only lists optional future ideas (live 3D replay, NPC LOD, more venues, real audio, prop
   atlas, AI mockups for the new catalogue entries, image-to-mesh props). GUT was consciously not