Draw prop models with a single atlas material

Each generated prop used one material per part (body, accent, metal...). The
model pipeline now packs a prop's palette into one power-of-two colour atlas plus a
matching metallic/roughness atlas and emits a single material:

- atlas_for(): 8x8 cells per palette role, colours in sRGB, roughness in the G and
  metallic in the B channel of the second atlas; NEAREST sampling keeps cells flat.
- build_glb(): every part merges into one primitive with constant per-vertex atlas
  UVs; the material uses baseColorTexture + metallicRoughnessTexture with factors 1.0.
- audit_glb(): fails unless a prop has exactly one material, both atlas textures,
  two images, and TEXCOORD_0 on every primitive.
- Removed the now-unused sRGB->linear factor helpers; colour travels in the texture.

Verification: headless 181 -> 182 (new "one material per prop scene" test), Python
tooling 53/53 (atlas determinism/size/UV/palette-override tests replace the old
factor tests), model audit OK for all six props, screenshots regenerated and
screen_designs.py compare still 6 good / 0 bad. Version 0.3.1.

TODO.md

@@ -1,7 +1,7 @@
 # TODO
 
-Godot 4.3 prototype. Latest release **v0.3.0** (rendering, model pipeline, selection pass and a
-Web build). Headless suite **181/181** plus **53** Python tooling tests; the session, PLAN-verb, crowd, emergency, timeline, audio, scenario-content, and
+Godot 4.3 prototype. Latest release **v0.3.1** (prop atlas). 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.
 
 All planned backlog sections are complete (see `DONE.md`). Optional future ideas live below.
@@ -14,6 +14,5 @@ All planned backlog sections are complete (see `DONE.md`). Optional future ideas
 - [ ] Real voice/dialogue audio (the crowd bed and radio crackle are procedural).
 - [ ] Swap the primitive prop builder for an image-to-mesh model API behind the same
       `assets/models/generated/<prop>.glb` contract.
-- [ ] Texture atlas for the props so they share one material instead of one per part.
 - [ ] Regenerate the AI screen mockups for the seven new catalogue entries (only the
       deterministic wireframes exist so far).