Document the frozen npc_base fallback boundary

docs/blender-assets.md

@@ -35,6 +35,16 @@ captures in `docs/screenshots/`.
   doors, stair handrails and a dark sedan. Today's in-game props are primitive
   boxes, and the trees are cones.
 
+## Frozen fallback boundary
+
+`assets/models/npc/npc_base.blend` is the tracked, frozen plain-primitive
+fallback. Prototype head/face/hand detail from the local part pipeline must not
+be saved into it; a separate contract decision (ADR or owner sign-off) is
+required before any prototype detail is absorbed. Working scenes live under
+`tmp/npc-parts/` (gitignored) and are exported as local GLBs for review, never
+directly over the tracked `.blend`. The same rule already appears in
+[`asset-tooling.md#recommended-part-workflow`](asset-tooling.md#recommended-part-workflow).
+
 ## Optional MCP
 
 Blender MCP can be used for interactive head/face/hand part exploration, but it