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.

README.md

@@ -32,8 +32,7 @@ The central design goal is simple:
 > Downloads are public (no sign-in required) since the host's instance-wide
 > `REQUIRE_SIGNIN_VIEW` was disabled on 2026-10-31.
 >
-> **Latest: v0.3.0** — rendering and NPC presentation pass, generated prop models, aim-free
-> selection with precision walk, and a Web (browser) build.
+> **Latest: v0.3.1** — props draw with a single atlas material instead of one material per part.
 > Releases are built automatically when a `vX.Y.Z` tag is pushed
 > (see [`docs/RELEASING.md`](docs/RELEASING.md)); manual builds use
 > `./tools/build_releases.sh`. Installer/AppImage packaging runs in the same pipeline
@@ -378,7 +377,7 @@ Implemented so far:
 - Godot 4.3 project scaffold, input map, procedural world builder
 - Venue-driven simulation core: `ShiftCoordinator`, `VenueScenario`, `EventScheduler`, `NPCController` + FSM, `AccessVerifier`, `EscalationManager`, `RadioSystem`, `IncidentRecorder`, `ReputationEvaluator`, `CrowdSystem`
 - Three authored scenarios (Hollywood Boulevard building, The Joke Shop, Late Night TV Studio) with access policies, NPC archetypes, and event pools
-- Headless unit tests: **181 passing** (access rules and venue-correct access loadout, escalation, radio cooldown/battery, NPC reaction determinism, door lock, flashlight perception, persistence round-trip, settings round-trip, navmesh, step colliders, skyline alpha, scenario data, full-shift simulation, wand screening, responder dispatch, localization strings, focus cone, action wheel, session shell, PLAN verbs and incident recording, crowd presentation, emergency aftermath, body-camera timeline, NPC theater poses, audio, accessibility, authored scenario content, string formatting, `.tres` venue round-trip, surface materials and environment, focus ranking and outfit/gait helpers, generated prop models, web export preset)
+- Headless unit tests: **182 passing** (access rules and venue-correct access loadout, escalation, radio cooldown/battery, NPC reaction determinism, door lock, flashlight perception, persistence round-trip, settings round-trip, navmesh, step colliders, skyline alpha, scenario data, full-shift simulation, wand screening, responder dispatch, localization strings, focus cone, action wheel, session shell, PLAN verbs and incident recording, crowd presentation, emergency aftermath, body-camera timeline, NPC theater poses, audio, accessibility, authored scenario content, string formatting, `.tres` venue round-trip, surface materials and environment, focus ranking and outfit/gait helpers, generated prop models, web export preset, prop atlas material)
 - Simulation extras: protester archetype, CALL_SOMEONE reaction, door lock/secure mechanic (visual indicator), flashlight with night perception bonus, LATE shift phase reaching the violent-threat event
 - First-person prototype with interaction HUD, access inspection, wand screening, radio report form (incl. emergency hold-to-confirm), dispatched responder avatars, aim-free targeting, venue cycling between all three scenarios, color-coded escalation feedback, procedural audio cues, navigation-mesh NPC paths, JSON-backed localized strings, persisted shift history, debug overlay, and shift review
 - Control pass: focus cone on NPC heads with a billboarded blue circle in front of the face and name, action wheel (LMB/Space) with de-escalation-first slices, mouse-look freeze while a panel is open, acceleration-based walking, and named input actions (`action_menu`, `confirm`, `cancel`, `inspect`, `wand`)
@@ -401,6 +400,7 @@ Implemented so far:
 - Visual polish pass (v0.2.0): phase-based lighting (sun/sky/ambient per shift phase), street lamps with night-mode bulbs and omni lights, glowing marquee signage, crosswalk, planters and low-poly street trees, NPC height/brightness variety, denser routine traffic — day and late-night screenshot sets
 - Assets pass: authored textures in `assets/textures/` (brick facade, concrete, asphalt, awning, skyline, vignette, HUD icons, access-card art) generated by `tools/generate_textures.py`, soft sun shadows, humanoid NPC rigs with archetype props + procedural walk bob and attention head-turn, inspection artifact cards, escalation vignette, mode icons, and a body-camera overlay (REC + shift clock)
 - Rendering pass (v0.3.0 work, [ADR-0012](docs/adr/0012-surface-materials-and-generated-props.md)): one material module with roughness/metallic profiles per surface, ACES tonemapping, glow, aerial fog and colour grading driven by the shift phase, correctly scaled brick, framed facade windows that light up at night, door trim and handles, a curb and lane dashes, and the glowing marquee/lamp bulbs. NPCs gained per-role clothing palettes, caps/hoods/backpacks/sunglasses/lanyards, hands and shoes, and an arm/leg walk swing
+- Prop atlas (v0.3.1): `tools/model_pipeline.py` packs each prop's palette into one power-of-two colour atlas plus a metallic/roughness atlas and emits a single material with per-vertex UVs, so a prop is one draw material rather than one per part; `audit` enforces the contract
 - Generated prop models (v0.3.0 work): `tools/model_pipeline.py` builds bollards, benches, bins, planters, newspaper boxes and a hydrant as `.glb` props with per-part materials, loaded by `WorldBuilder` with a procedural fallback; the optional AI concept step (`model_pipeline.py concept`) supplies per-prop colour palettes
 - Web build: `build/step-line-web-<version>.zip` (Godot's non-threaded web template, no COOP/COEP headers needed) built by `tools/build_releases.sh` and published with the desktop artifacts; serve the unzipped folder from any static host
 - Aim-free selection pass (v0.3.0 work, [ADR-0011](docs/adr/0011-aim-free-selection-and-precision-walk.md)): the focus cone ranks candidates by angle then distance, `Tab` and the mouse wheel cycle nearby people nearest-first and pin the selection, right click clears focus with a short re-acquire grace, the HUD reports how many people are in the ring, and holding `Shift` walks at half speed for close work