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.

MEMORY.md

@@ -31,6 +31,12 @@ Session memory for durable decisions, state, and gotchas. Keep concise, remove o
   (static hosting, no COOP/COEP); `tools/build_releases.sh` exports `build/web/` and zips it to
   `build/step-line-web-<ver>.zip`, and `publish_release.sh` uploads it. Covered by
   `src/test/tests/test_web_export.gd`.
+- **Prop atlas (v0.3.1):** `model_pipeline.atlas_for()` builds a per-prop power-of-two PNG with
+  one 8×8 cell per palette role, plus a metallic/roughness atlas (G=roughness, B=metallic). The
+  GLB merges all parts into one primitive with constant per-vertex UVs (cell centre) and a single
+  material (`baseColorTexture` + `metallicRoughnessTexture`, NEAREST sampling). Colours are sRGB in
+  the texture, so the old `srgb_to_linear`/`_linear_rgba` helpers were removed. `audit_glb` fails
+  if a prop is not exactly one material or misses TEXCOORD_0/images.
 - **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