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.

MEMORY.md

@@ -492,3 +492,9 @@ Scope: facts and traps that cost time to establish. Planned work lives in [TODO.
 - **ComfyUI is reachable at `127.0.0.1:8188` (0.44.0)** and its API is the clean way in: `POST /prompt` with an API-format graph (`[node_id, output_index]` references — the UI's workflow JSON cannot be posted), then poll `/history/<id>` and fetch `/view?filename=...`. `Tools/comfyui_generate.py` does this and writes a sidecar with model, prompt, seed and the full graph. Installed checkpoints: `juggernautXL_ragnarok.safetensors`, `epicrealismXL_pureFix.safetensors`, `sd-v1-5-inpainting.ckpt`, plus inpainting variants.
 - **Do not ask Stable Diffusion for a flat texture swatch.** Three attempts at "seamless skin texture" produced an anatomical close-up twice and a leather-scale surface once. SDXL has no reliable notion of a material sample without anatomy, so every seed needs manual rejection — worse than noise we control. Procedural generation won for skin pores (`Tools/generate_character_textures.py`, tileable octave noise) and a strand sheet for hair.
 - **Unreal collapses the second material slot on a skeletal-mesh import.** A joined hair cap (Blender shows two slots, hair faces on index 1) arrives as a single slot named after the body material, so the hair renders with the skin material. `unreal.SkeletalMesh` has no `set_material`, so it cannot be re-slotted after import. A separate skeletal mesh attached as a second component is the way around it.
+
+## 2026-11-09 — Hair as a separate mesh, and a material-editing crash
+
+- **Hair joined into the body mesh loses its material.** Blender shows two slots (hair faces on index 1) and the FBX exports both, but Unreal's skeletal-mesh import arrives with **one** slot named after the body material, so the hair renders as skin. `unreal.SkeletalMesh` has no `set_material`, so it cannot be re-slotted afterwards. A separate skeletal mesh imported on its own keeps its slot and imports cleanly (`CharacterHair` → `M_Character_Hair`).
+- **A second skeletal component follows the body with `SetLeaderPoseComponent(GetMesh())`** — no second rig, no second animation, and it hides with the body in first person.
+- **`unreal.MaterialEditingLibrary` segfaults in `-run=pythonscript`** when it clears and rebuilds a material graph. The tool ran its texture imports, then crashed the commandlet with no Python traceback. Graph editing needs an editor session, or a material created from scratch rather than edited in place.