auto-git:

[add] src/geometry/model-instance-collider-generation.ts
 [add] src/runtime-three/rapier-collision-world.ts
 [change] AGENTS.md
 [change] CHAT_CONTEXT.md
 [change] architecture.md
 [change] package.json
 [change] prompts-lite.txt
 [change] prompts.txt
 [change] roadmap.md
 [change] src/assets/model-instances.ts
 [change] src/document/migrate-scene-document.ts
 [change] src/document/scene-document-validation.ts
 [change] src/document/scene-document.ts
 [change] src/runtime-three/first-person-navigation-controller.ts
 [change] src/runtime-three/navigation-controller.ts
 [change] src/runtime-three/runtime-host.ts
 [change] src/runtime-three/runtime-scene-build.ts
 [change] src/runtime-three/runtime-scene-validation.ts
 [change] src/viewport-three/viewport-host.ts
 [change] testing.md

roadmap.md

@@ -342,6 +342,40 @@ The tool becomes more than brush-only by supporting imported assets, authored li
 
 ---
 
+### Slice 3.1B — Imported model collider authoring
+
+#### Deliverables
+
+- canonical collision settings on model instances
+- explicit collision mode selector for:
+  - none
+  - terrain
+  - static
+  - dynamic
+  - simple
+- collision visibility/debug toggle
+- generated collider data derived from imported model geometry plus authored settings
+- Rapier-backed collision/query integration for imported-model colliders and broad-phase/narrow-phase handling
+- runtime build support for imported-model colliders in the runner collision path
+- editor/runner debug visualization of generated collision where enabled
+
+#### Acceptance criteria
+
+- author can choose a collision mode per model instance in the inspector
+- imported models can participate in runner collision without hand-authored invisible geometry
+- collider modes mean exactly:
+  - none = no collider
+  - terrain = heightfield collider, static only
+  - static = triangle mesh collider, fixed only
+  - dynamic = convex decomposition into compound collider, dynamic/kinematic capable
+  - simple = one cheap primitive or one convex hull
+- generated collision survives save/load through canonical settings and deterministic rebuild behavior
+- broad-phase and narrow-phase pruning are handled by Rapier instead of app-specific custom collision code
+- the slice does not require a full gameplay-physics sandbox
+- unsupported or partial modes fail clearly instead of silently pretending to work
+
+---
+
 ### Slice 3.2 — Local lights and skyboxes
 
 #### Deliverables
@@ -593,6 +627,7 @@ A slice is complete only when:
 - per-face UV persistence
 - picking accuracy
 - collision generation from brush data
+- collision generation from imported model data
 - imported asset/material compatibility
 - browser audio unlock behavior
 - input edge cases across browsers