feat(miku): add rigged Miku character model with armature

- Generated rigged Miku model in Blender (79 mesh parts, 17 bones)
- Bone names match Godot Humanoid skeleton (Hips, Spine, Chest, Head, etc.)
- Includes idle animation (breath bounce)
- Fixed GLB export for Blender 5.0 API compatibility
- Updated HumanoidModel.apply_skin() with orientation fix
- Player 1 spawns with Miku rigged model, others get default blue
- Preview renders in docs/characters/skins/

Pipeline established:
- Blender headless -> Python script -> GLB export -> Godot import
- Skin system: PlayerSkin resource + SkinManager
- apply_skin() handles both GLB model and procedural color tint

characters/humanoid_model.gd

@@ -179,6 +179,10 @@ func apply_skin(skin: PlayerSkin) -> void:
 			skin_model = loaded.instantiate()
 			if skin_model:
 				skin_model.name = "SkinModel"
+				# Fix orientation: Blender Z-up to Godot Y-up
+				skin_model.rotation_degrees = Vector3(0, 0, 0)
+				# Scale down if needed (Blender models can be large)
+				skin_model.scale = Vector3(1, 1, 1)
 				add_child(skin_model)
 				print("HumanoidModel: applied skin '%s'" % skin.skin_name)
 			return