Load and animate the Blender-authored NPC base mesh

Wire the skinned model from ADR-0013 into NpcAvatar:

- _build_skinned_body() instantiates the imported npc_base.glb, binds the 14
  armature bones, and keeps the procedural _build_body() as the offline
  fallback (use_skinned_model can force it in tests)
- outfit_for()/layer_for() tint the five material slots through the new
  SurfaceMaterials.npc_slot(); accessories and hand items attach under Rig/Head
- the existing limb pivots (Hip/Knee/Shoulder/Elbow/Hand + Head marker) keep the
  old animation math; _sync_skeleton() copies their rotations to bones. Bone
  pose rotation is absolute local orientation, so it is composed with the rest
  basis (otherwise the whole body snaps into a broken pose)
- skinned rigs scale uniformly (non-uniform parent scale shears the skin) and
  keep Rig/Head for the focus cone, nametag, and reaction tag
- commit the editable npc_base.blend plus a .gdignore: Godot's editor otherwise
  tries to import the blend and hangs during --import
- npc_pipeline audit tests (contract, textures, clean GLB, committed source)

Verification: headless 212/212, Python tooling 60/60, NPC budget 16 avatars =
144 meshes at ~6.9 ms, movement/focus/props smokes pass, compare 11 good / 0 bad.
TODO Phases 0-3 checked; environment lift and release remain.

docs/plan-graphics-v0.6.md

@@ -69,7 +69,10 @@ From `src/game/npc_avatar.gd` and `src/game/guard_controller.gd`:
 
    Record the working invocation in `MEMORY.md` (the Im-Dornäcker notes already
    warn about the cwd gotcha).
-2. Freeze the asset contract in writing (this document + ADR-0013):
+2. Freeze the asset contract in writing (this document + ADR-0013). No
+   animation clips are exported: `_animate()` and `_idle_layer()` drive the
+   armature bones directly at runtime, so the GLB only needs the skinned rest
+   mesh, the armature, and the material maps.
    - `assets/models/generated/npc_base.glb` — one skinned mesh (or a small fixed
      set of sub-meshes: body / head / hands) with a **named armature** and UVs.
    - Material slots rather than one atlas-per-prop: `skin`, `cloth`,