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.

TODO.md

@@ -1,44 +1,15 @@
 # TODO
 
-Godot 4.3 prototype. Latest release **v0.3.3** (staged captures for the new screens). Headless
-suite **182/182** plus **53** Python tooling tests; the session, PLAN-verb, crowd, emergency,
-timeline, audio, scenario-content, and windowed focus/radial smokes pass. Anonymous downloads are
-enabled on the host.
+Godot 4.3 prototype. Latest release **v0.4.0** (prop collision, placement, variants). Headless
+suite **187/187** plus **53** Python tooling tests; the session, PLAN-verb, crowd, emergency,
+timeline, audio, scenario-content, props, and windowed focus/radial smokes pass. Anonymous
+downloads are enabled on the host.
 
-The three sections below are the planned presentation pass: props, NPC characters, and player
-movement. Each task is meant to be a small, reviewable slice with its own verification.
+The sections below are the rest of the presentation pass: prop pipeline checks, NPC characters,
+and player movement. Each task is meant to be a small, reviewable slice with its own verification.
 
 ## Better Props
 
-### Collision and placement
-
-- [ ] Give every generated prop a real collider. Add a simple box/cylinder collision shape to the
-      `.glb` (or emit a `physics` block per part that `WorldBuilder` turns into a
-      `StaticBody3D`), so benches, bins, planters, hydrants, and newspaper boxes cannot be walked
-      through.
-- [ ] Add a windowed check that the guard and NPCs path around props instead of through them, and
-      that the fallback primitive props get the same colliders as the modelled ones.
-- [ ] Replace the hard-coded prop positions in `_build_street_detail()` with a placement table
-      (zone, offset, rotation, variant) so clusters read as street furniture: entrance pair,
-      curb line, mid-block, wall side. Assert no overlaps with the door, steps, or spawn lane.
-- [ ] Reuse one `PackedScene` per prop type but expose deterministic variants (rotation, scale,
-      optional accessory such as a chain, sign, or bag) so repeated props stop looking cloned.
-
-### New props and detail
-
-- [ ] Add street-sign props: parking sign, no-parking sign, one-way sign, and a small post
-      (text via the atlas, not a texture per sign).
-- [ ] Add a bus-stop shelter with a bench, roof, and side panel; a bike rack; a mailbox; and a
-      phone booth or newspaper stand.
-- [ ] Add a traffic signal (pole, arm, three lenses) and a street lamp variant that matches the
-      lamps placed by `WorldBuilder`.
-- [ ] Add cafe/shop clutter: sandwich board, stacked crates, café table + two chairs, and a tied
-      trash-bag pile for the Joke Shop edge.
-- [ ] Detail the existing six props: chamfered edges on boxes, rim rings on bollard and bin,
-      bolts on the hydrant, bench slats with gaps, visible soil plus foliage on the planter.
-- [ ] Add a close-range detail layer: pack a subtle grunge/AO strip into the atlas (or a shared
-      detail normal) so props do not read as flat-coloured boxes at conversation distance.
-
 ### Pipeline and checks
 
 - [ ] Extend `audit_glb()` with a triangle budget, degenerate-triangle test, `TEXCOORD_0` inside