Prepare the modular part intake: inventory, extract, verify, join and the part contract

assets/models/blender/manifest.json

@@ -567,7 +567,7 @@
         "min": [
           -0.262,
           -0.004,
-          -0.091
+          -0.092
         ]
       },
       "category": "npc",
@@ -597,7 +597,7 @@
         "hair"
       ],
       "triangles": 6242,
-      "vertices": 13100
+      "vertices": 13110
     },
     "npc_crowd_d": {
       "base": "worker",

assets/models/blender_src/quaternius/parts/README.md

@@ -0,0 +1,54 @@
+# Quaternius modular parts
+
+Build inputs for outfit, hair and accessory variety. Committed like the three whole-body
+sources, so `build` works from a fresh checkout without the upstream pack.
+
+## Where they come from
+
+The upstream *Ultimate Modular Characters* pack is distributed as a Google Drive folder
+(<https://drive.google.com/drive/folders/1USAAquX2JJWuA2m6zol0KUkFe3UkZ8zX>, CC0 1.0). It cannot be
+fetched reproducibly by a script, so the owner downloads it **once** into the gitignored
+`tmp/quaternius-intake/`. Everything after that is scripted:
+
+```sh
+python3 tools/quaternius_parts.py inventory --dir tmp/quaternius-intake
+python3 tools/quaternius_parts.py extract \
+  --file tmp/quaternius-intake/<file>.glb --mesh <mesh> --part <part>
+python3 tools/quaternius_parts.py verify --dir assets/models/blender_src/quaternius/parts
+```
+
+`inventory` and `verify` run outside Blender; `extract` needs Blender, because it rewrites the skin.
+The pack itself is never committed — only the parts that are actually used, each with its SHA-256
+recorded here and in `CREDITS.md`.
+
+## Part contract
+
+A part is **one skinned mesh on the 62-joint rig**:
+
+- one mesh object, parented to the rig's armature, carrying `JOINTS_0`/`WEIGHTS_0`;
+- the standard joint names (`Hips`, `Chest`, `Head`, `UpperArm.L`, …). The rig check in
+  `quaternius_parts.py verify` fails a part that is missing any of them, because
+  `NpcAvatar` maps those bones;
+- materials named after a contract slot (`skin`, `cloth`, `pants`, `hair`, `accessory`) so the
+  runtime tint reaches them, or a fixed-colour detail name that is deliberately not tinted;
+- triangles counted against the character budget: base body plus every joined part must stay
+  inside 24,000 triangles and 24,000 glTF vertices.
+
+The builder drops the part's own armature when it joins the mesh onto a base body, because the base
+already owns the skeleton and two armatures would duplicate every bone. Weights survive the join
+because Blender merges vertex groups by name.
+
+## Assigning parts to a role
+
+```python
+"npc_vest": {"layer": "vest", "base": "worker", "parts": ["hivis_vest"], ...}
+```
+
+`build` joins those parts after the body join and before the material slot map, so the part's
+materials go through the same slot mapping as the base. The T-pose cache is keyed on the part list.
+
+## Kept parts
+
+| Part | Source file | SHA-256 | Tris |
+| --- | --- | --- | --- |
+| _(none yet — the pack has not been intaken)_ | | | |