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.cpp

@@ -90,6 +90,22 @@ AGigLineCharacter::AGigLineCharacter()
 	GetMesh()->SetCollisionEnabled(ECollisionEnabled::NoCollision);
 	GetMesh()->SetHiddenInGame(true);
 
+	// Hair: a separate component that copies the body's pose, so it follows the head
+	// bone without a second rig or a second animation. Hidden with the body.
+	HairMesh = CreateDefaultSubobject<USkeletalMeshComponent>(TEXT("HairMesh"));
+	HairMesh->SetupAttachment(GetCapsuleComponent());
+	HairMesh->SetCollisionEnabled(ECollisionEnabled::NoCollision);
+	HairMesh->SetHiddenInGame(true);
+	static ConstructorHelpers::FObjectFinder<USkeletalMesh> HairFinder(
+		TEXT("/Game/Test/Character/CharacterHair.CharacterHair"));
+	if (HairFinder.Succeeded())
+	{
+		HairMesh->SetSkeletalMesh(HairFinder.Object);
+		HairMesh->SetRelativeLocation(FVector(0.0f, 0.0f, -87.5f));
+		HairMesh->SetRelativeRotation(FRotator(0.0f, -90.0f, 0.0f));
+		HairMesh->SetLeaderPoseComponent(GetMesh());
+	}
+
 	// Third-person boom. bDoCollisionTest is the camera-collision feature: the arm
 	// shortens when something is between the pivot and the desired camera position.
 	ThirdPersonArm = CreateDefaultSubobject<USpringArmComponent>(TEXT("ThirdPersonArm"));
@@ -795,6 +811,10 @@ void AGigLineCharacter::ToggleCamera()
 	FirstPersonCamera->SetActive(!bThirdPerson);
 	ThirdPersonCamera->SetActive(bThirdPerson);
 	GetMesh()->SetHiddenInGame(!bThirdPerson);
+	if (HairMesh)
+	{
+		HairMesh->SetHiddenInGame(!bThirdPerson);
+	}
 	if (bThirdPerson)
 	{
 		UpdateBodyAnimation();