feat: add rigged+animated Miku player model with auto-rig pipeline

- Add miku_rigged_animated.glb (3.27MB) with 18-bone skeleton and 6 animations
  (Idle, Walk, Run, Jump, Crouch, Death) via Blender auto-rig script
- Add tools/rig_and_animate.py: reusable Blender script for rigging any humanoid
  mesh with Mixamo-compatible bone naming
- Update SkinnedPlayerModel: scale_factor, first-person mode, animation state
  matching (Idle/Walk/Run/Jump/Crouch/Death)
- Update level_runtime.gd: use skinned model for local player, procedural
  humanoid for remote players
- Update skin_manager.gd and test_level_builder.gd to use animated model
- Fix Godot 4.2.1 'is not Type' syntax in 5 weapon files
- Add editor/import_miku_to_tscn.gd for editor-based GLB import

movement/player_movement_controller.gd

@@ -971,6 +971,9 @@ func die(impulse: Vector3 = Vector3.ZERO) -> void:
 	var humanoid = get_node_or_null("HumanoidModel")
 	if humanoid:
 		humanoid.visible = false
+	var skinned = get_node_or_null("SkinnedModel")
+	if skinned:
+		skinned.visible = false
 		
 	# Disable collision so player doesn't block bullets
 	var col = get_node_or_null("CollisionShape3D")