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.

Tools/ue_character_hair_import.py

@@ -0,0 +1,70 @@
+# Import the hair cards as their own skeletal mesh (0.2.2).
+#
+#   blender -b Art/blender/character/character-2026-11-07.blend \
+#       --python Tools/character_hair_build.py -- --out /tmp/gigline/character-hair.fbx
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/ue_character_hair_import.py -nullrhi -unattended -nopause -stdout
+#
+# The hair is a separate asset on purpose: Unreal collapses the second material slot
+# when hair is joined into the body mesh, so a card system has to be its own object
+# (which is what the character-artist skill asks for anyway).
+#
+# Result file: /tmp/gigline/character-hair-import.txt
+
+import unreal
+
+SRC = "/tmp/gigline/character-hair.fbx"
+DEST = "/Game/Test/Character"
+NAME = "CharacterHair"
+RESULT = "/tmp/gigline/character-hair-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def main():
+    task = unreal.AssetImportTask()
+    task.filename = SRC
+    task.destination_path = DEST
+    task.destination_name = NAME
+    task.automated = True
+    task.save = True
+    task.replace_existing = True
+
+    ui = unreal.FbxImportUI()
+    ui.import_mesh = True
+    ui.import_as_skeletal = True
+    ui.import_animations = False
+    ui.import_materials = True
+    ui.import_textures = False
+    ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+    task.options = ui
+    unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+
+    mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{NAME}")
+    if mesh is None:
+        log(f"MISSING {DEST}/{NAME}")
+        lines.append("VERDICT: FAIL")
+        return
+    mats = mesh.get_editor_property("materials")
+    log(f"{NAME}: {len(mats)} material slot(s)")
+    for i, m in enumerate(mats):
+        face = m.get_editor_property("material_interface")
+        log(f"  slot {i}: {face.get_name() if face else 'None'}")
+    lines.append("VERDICT: PASS")
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+    lines.append("VERDICT: FAIL")
+
+unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+with open(RESULT, "w") as f:
+    f.write("\n".join(lines) + "\n")