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.

Game/GigLine/Source/GigLine/GigLineCharacter.h

@@ -155,6 +155,17 @@ protected:
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Camera")
 	float ThirdPersonBoomLength = 340.0f;
 
+	/**
+	 * Hair, as its own skeletal mesh following the body's pose.
+	 *
+	 * A second component rather than geometry joined into the body: Unreal collapses
+	 * the second material slot on a skeletal-mesh import, so joined hair rendered with
+	 * the skin material. The character-artist skill calls for hair cards to be their
+	 * own mesh for the same reason it calls for swappable parts to be separate.
+	 */
+	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "Body")
+	TObjectPtr<USkeletalMeshComponent> HairMesh;
+
 	UPROPERTY()
 	TObjectPtr<UAnimSequence> IdleAnimation;