feat(pipeline): grow skirt and hair bone chains for a costume that has none

Miku shipped with 0 cloth chains against Taila's 35, so her twin tails
hung off her skull like a helmet. Nothing downstream could fix it: the
spring solver simulates cloth BONES, and a garment with none is welded to
whatever body bone it was weighted to. Every auto-rigged model is in that
state, and her source was an unrigged mesh.

tools/cloth_bones.py builds them, which is the job a technical artist does
by hand on a model like this. It finds the geometry by MATERIAL SLOT — the
artist already answered which surface is hair, and on a joined mesh (what
the auto-rig leaves behind) the slot is the only separation left. Hair is
split into connected islands, because a strand is a connected piece of
surface and clustering by position would merge two ponytails passing near
each other. A skirt is split into radial wedges instead, because a skirt
is ONE connected surface and islands would return the whole thing as a
single piece — the bell-shaped failure. Each clump gets a polyline fitted
down its middle by binning vertices by distance and taking centroids, so
the chain follows the piece's own curve rather than cutting the corner on
a bend, and vertices are re-weighted onto it while the first 22% keeps its
original body weight so the scalp stays on the skull.

On Miku: 19 chains, 57 bones from one `hair` slot. Sidecar 0 -> 19 chains.
Idle stability 0.007-0.018 deg/frame. Mesh intact, verified by render.

Opt-in, via `pipeline.py --grow-cloth`, and run before the retarget so
describe_rig() finds the chains by name exactly as it would an artist's.

Known limits, recorded in the skill: it cannot find a garment sharing a
material with the body (Miku's skirt is on her `body` slot, so she got
hair and no skirt), and grown chains are a fallback — an artist's chains
carry intent that no geometric fit recovers.

Co-Authored-By: Claude Opus 5 <[email protected]>

.claude/skills/character-pipeline/SKILL.md

@@ -42,6 +42,10 @@ python tools/pipeline.py --uid <uid> --name <name>
 
 # From an unrigged mesh — auto-rigs, and accepts the quality loss
 python tools/pipeline.py --input <mesh.glb> --name <name>
+
+# Rigged (or auto-rigged) but with NO skirt/hair bones — grow them, or the
+# costume is welded solid and the spring solver has nothing to simulate
+python tools/pipeline.py --input <model.glb> --name <name> --rigged --grow-cloth
 ```
 
 Then, once, so Godot sees the new files:
@@ -64,6 +68,7 @@ Blender is required (`BLENDER_PATH`, or auto-found under
 | Fix unlit/emissive materials | `tools/gltf_fix.py` | Textures surviving import at all |
 | Resolve bone ROLES, not names | `tools/rig_map.py` | The model's own skeleton |
 | Rebuild parenting | `retarget.py::rebuild_hierarchy` | Limbs/cloth following the hips |
+| Grow cloth chains (opt-in) | `tools/cloth_bones.py` | A costume that has no bones being able to move at all |
 | Subdivide cloth panels | `retarget.py::subdivide_cloth_panels` | A skirt being able to bend at all |
 | Retarget clips as rest-relative deltas | `retarget.py::retarget_clip` | Limbs not being twisted by foreign bone roll |
 | Leave cosmetic bones unkeyed | `export_optimize_animation_keep_anim_armature=False` | Physics owning the cloth |
@@ -79,6 +84,8 @@ is a list of things that cost a debugging cycle to learn.
 - **`references/separation.md`** — body vs. garments vs. hair: what must stay
   separate, how cloth chains are detected and classed, why cloth is never
   skinned to a leg, and the ZZZ-convention mapping.
+- **`references/growing-cloth-bones.md`** — what to do when the source has no
+  skirt or hair bones: how the chains are fitted to the geometry and re-weighted.
 - **`references/rigging.md`** — role resolution, hierarchy rebuild, cloth panel
   subdivision, twist bones, joint helpers, the retarget maths.
 - **`references/cloth-and-hair.md`** — the position-based spring solver, its