feat(hair): hair cards as their own skinned mesh, attached as a second component

Follows the character-artist skill on two points it is explicit about: hair for a
game character is a card system, and swappable parts stay separate meshes.

- Tools/character_hair_build.py builds SM_Char_Hair: a scalp cap plus 18 cards in
  three rings, quads only, every vertex weighted 1.0 to the head bone, UV-mapped for
  the strand sheet from Tools/generate_character_textures.py. 321 verts / 404 tris.
- Tools/ue_character_hair_import.py imports it as its own skeletal mesh, which is the
  point: joined into the body, Unreal collapses the second material slot and the hair
  renders with the skin material.
- The pawn carries it as a second USkeletalMeshComponent on SetLeaderPoseComponent, so
  it follows the head bone with no second rig and hides with the body in first person.

Still wrong and recorded: the cards point outward, so the silhouette reads as a spiked
crown rather than a short cut. The next pass orients them tangentially along the
skull with a second overlapping row. Tools/ue_character_materials.py also segfaults in
the headless commandlet when editing a material graph, so the generated textures are
not wired yet and the hair keeps its Blender base colour.

TODO.md

@@ -15,10 +15,12 @@ Actionable task list for this project. This file is the single, central place fo
 
 - [x] Restore the stride capture after the pose rewrite. The walk cycle started on a **sine**, zero at both the start and the middle of the cycle, so the deterministic capture kept landing on the two passing frames where the legs are together. Starting the cycle at maximum stride (cosine) fixed it with no other change; `demo-third-person-walk` is back as shot 8 and shows a genuine stride.
 
-- [A] Tune skin shading and hair materials on the player character. **Tooling in place 2026-11-09, hair blocked on an engine-side limit:**
+- [~] Tune skin shading and hair materials on the player character. **Tooling in place 2026-11-09, hair blocked on an engine-side limit:**
   - `Tools/comfyui_generate.py` drives the local ComfyUI instance (`127.0.0.1:8188`) for textures, writing a JSON sidecar with the model, prompt, seed and full workflow so anything generated can be regenerated or rejected. Installed models include `juggernautXL_ragnarok.safetensors` and `sd-v1-5-inpainting.ckpt`.
   - **Stable Diffusion is not usable for flat skin swatches.** Three attempts on two models produced an anatomical close-up twice and a leather-scale surface once; SDXL has no reliable notion of "texture without anatomy". Skin detail is therefore generated procedurally instead (`Tools/generate_character_textures.py`: tileable octave noise for pores plus a strand-sheet texture for hair).
-  - **Hair geometry is built but not shipped.** `add_hair_cap()` creates a cap conforming to the head and weights it to the `head` bone (Blender shows two material slots, hair faces on index 1), but Unreal's import **collapses the second material slot**, so the cap renders with the skin material and reads as a bald skull. It is behind `--hair`, off. Next step: export the hair as its own skeletal mesh and attach it as a second component, or bake the hair colour into vertex colours.
+  - **Hair is now its own asset** (`Tools/character_hair_build.py` → `SM_Char_Hair` → `/Game/Test/Character/CharacterHair`), which is what the `character-artist` skill asks for: cards are a separate mesh, not geometry joined into the body. It imports with its own material slot and is attached in C++ as a second `USkeletalMeshComponent` on `SetLeaderPoseComponent`, so it follows the head bone without a second animation. 321 verts, 192 quads, 404 tris.
+  - **The card layout is still wrong.** The cards are anchored pointing outward, so the silhouette reads as a spiked crown instead of a short cut. Next iteration: orient each card tangentially along the skull, add a second overlapping row, and drive the shape from the scalp normal rather than a fixed outward vector.
+  - `Tools/ue_character_materials.py` (wiring the generated textures into the skin and hair materials) **segfaults in the headless commandlet** when it edits the material graph. Not wired; the hair keeps the dark base colour it carries from Blender. Needs either an editor-session run or a material asset built from scratch instead of edited in place.
 
 - [ ] Tag `v0.2.2` and write `CHANGELOG.md` entry; then remove this section.