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

docs/blender-assets.md

@@ -45,6 +45,44 @@ required before any prototype detail is absorbed. Working scenes live under
 directly over the tracked `.blend`. The same rule already appears in
 [`asset-tooling.md#recommended-part-workflow`](asset-tooling.md#recommended-part-workflow).
 
+## Local part pipeline
+
+The overhaul uses `tools/npc_parts.py` to build, extract, assemble, inspect and sheet the
+reusable head/face and hand parts before they are rolled into the 12 characters. It reuses the
+`blender_assets.py` builders (no second face/hand implementation) and stays local-only/headless.
+Part sources and preview GLBs live under `tmp/npc-parts/<variant>/`; they are **not shipped
+assets** — only the assembled character GLBs, AO/maps, the manifest and the review sheets are
+committed.
+
+```bash
+# Build one part fresh from the current builders (head|face|hand_l|hand_r)
+flatpak run org.blender.Blender --background \
+  --python "$PWD/tools/npc_parts.py" -- build --part face --variant npc_casual \
+  --out "$PWD/tmp/npc-parts/npc_casual/face.glb"
+# Pull the current shipped head/hands out of the built character GLB (comparison)
+flatpak run org.blender.Blender --background \
+  --python "$PWD/tools/npc_parts.py" -- extract --part hand_l --variant npc_casual \
+  --out "$PWD/tmp/npc-parts/npc_casual/hand_l.glb"
+# Assemble a variant (currently rebuilds the procedural character)
+flatpak run org.blender.Blender --background \
+  --python "$PWD/tools/npc_parts.py" -- assemble --variant npc_casual \
+  --out "$PWD/tmp/npc-parts/npc_casual/assembled.glb"
+# Inspect a part or assembled GLB (Blender import view)
+flatpak run org.blender.Blender --background \
+  --python "$PWD/tools/npc_parts.py" -- inspect --file "$PWD/tmp/npc-parts/npc_casual/face.glb"
+# Contact sheet / before-after comparison (outside Blender; stdlib + Pillow)
+python3 tools/npc_parts.py sheet --dir tmp/npc-parts/npc_casual \
+  --out docs/blender_previews/sheet_parts.jpg
+python3 tools/npc_parts.py compare --before DIR --after DIR --out sheet.jpg
+```
+
+`build --part head` produces the bare head shell; `build --part face` is the shell plus
+nose/eyes/mouth/ears; `build --part hand_l|hand_r` are the left/right hand geometry. `extract`
+pulls the current shipped geometry out of the built character GLB by vertex group (`Head` for
+`head`/`face`, `HandL`/`HandR` for the hands), so it shows what is actually shipped rather than
+what the builders would emit fresh. The assembled GLB must still pass
+`blender_assets.py inspect`/`audit`/`fit` before any part is accepted.
+
 ## Optional MCP
 
 Blender MCP can be used for interactive head/face/hand part exploration, but it