Add the local npc_parts.py head/face/hand part pipeline with tests and docs

docs/asset-tooling.md

@@ -430,6 +430,12 @@ scene, and connect from the sidebar. If the client cannot reach the addon:
 
 ### Recommended part-workflow
 
+The headless part pipeline is `tools/npc_parts.py` (see
+[`blender-assets.md#local-part-pipeline`](blender-assets.md#local-part-pipeline)). It builds,
+extracts, assembles, inspects and sheets the reusable head/face and hand parts and reuses the
+`blender_assets.py` builders, so there is no second face/hand implementation. The MCP steps below
+remain optional interactive exploration; the headless commands stay the build and audit path.
+
 1. Create a local scene, for example
    `tmp/npc-parts/npc_casual/head.blend`, or open a copy of an exported part GLB.
    Never save over the tracked `assets/models/npc/npc_base.blend`.
@@ -460,7 +466,7 @@ scene, and connect from the sidebar. If the client cannot reach the addon:
 - MCP is **not** part of `project.godot`, the build, CI, tests, release packages
   or runtime.
 - Do not commit `.blend` part scenes, MCP configuration, API keys or generated
-  previews. The working set stays in `tmp/`.
+  previews. The working set stays in `tmp/`; a part GLB is not a shipped asset.
 - Do not use MCP to bypass the contract, budgets, AO/UV rules or licence gate.
 - The 13 core bones must remain at their rest positions; extra optional bones
   are allowed only if `NpcAvatar` and the audit/tests still pass.