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

.agents/skills/gig-line-blender-asset-contract/references/verification.md

@@ -24,6 +24,7 @@ Blender must be absolute.
 | NPC base | `python3 tools/npc_pipeline.py audit` | exit 0, contract bones/slots/maps |
 | Proportions | `python3 tools/npc_proportions.py report` | all eight inside the stylized guide |
 | Single incoming GLB | `flatpak run org.blender.Blender --background --python "$PWD/tools/blender_assets.py" -- inspect --file "$PWD/path.glb" --category <cat> --json` | contract result and measurements |
+| Part GLB (local) | `flatpak run org.blender.Blender --background --python "$PWD/tools/npc_parts.py" -- inspect --file "$PWD/tmp/npc-parts/<variant>/<part>.glb"` | measurements; a part GLB is not a shipped asset |
 | Prop audit | `python3 tools/model_pipeline.py audit` | exit 0 when generated props changed |
 | Texture audit | `python3 tools/asset_pipeline.py audit assets/textures/ai/game_ready` | exit 0 when textures changed |
 
@@ -98,6 +99,12 @@ without measuring.
 
 ## Visual review criteria
 
+The part pipeline (`tools/npc_parts.py`) builds/extracts head/face/hand parts under
+`tmp/npc-parts/<variant>/`; these are local working files and are **not shipped**. Only the
+assembled character GLBs, AO/maps, the manifest and the review sheets are committed. An
+assembled GLB still goes through `blender_assets.py inspect`/`audit`/`fit` before any part is
+accepted. The `npc_base.blend` fallback is frozen: never save prototype detail into it.
+
 For each changed variant, inspect at least:
 
 - 2 m portrait: eyes, mouth, ears, neck/shoulder, hair/headgear clearance.