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.

DONE.md

@@ -661,3 +661,20 @@ produces a 63 MB static bundle and it serves locally with HTTP 200s for html/was
 - [x] Cover the preset, the non-threaded choice, and the release wiring in
       `src/test/tests/test_web_export.gd`.
 - [x] Document the artifact in `docs/RELEASING.md`, the README gallery/status, and `PLAN.md`.
+
+## Prop Texture Atlas
+
+Completed 2026-10-31 (v0.3.1). Headless **182/182**, Python tooling **53/53**; screenshots
+regenerated and `screen_designs.py compare` still reports 6 good / 0 bad.
+
+- [x] `model_pipeline.atlas_for()` packs each prop's palette roles into one small power-of-two
+      PNG and a matching metallic/roughness atlas (roughness in G, metallic in B), with nearest
+      sampling so a palette cell stays a flat colour.
+- [x] `build_glb()` merges every part into a single primitive with per-vertex atlas UVs and one
+      material (`baseColorTexture` + `metallicRoughnessTexture`), so a prop is one draw material
+      instead of one per part.
+- [x] `audit_glb()` checks the single-material/atlas contract (one material, both textures,
+      TEXCOORD_0 on every primitive, two images).
+- [x] Tests: atlas determinism and power-of-two sizing, UV range, palette override reaching the
+      atlas, and a Godot test that each imported prop scene uses exactly one material.
+- [x] Removed the now-unused sRGB→linear factor helpers (colours travel in the sRGB texture).