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

characters/skin_manager.gd

@@ -39,7 +39,7 @@ func _register_default_skins() -> void:
 	var miku_skin = PlayerSkin.new()
 	miku_skin.skin_name = "Miku"
 	miku_skin.description = "Hatsune Miku — Virtual Idol"
-	miku_skin.model_path = "res://assets/characters/skins/miku.glb"
+	miku_skin.model_path = "res://assets/characters/skins/miku_rigged_animated.glb"
 	miku_skin.color_tint = Color(0.0, 0.75, 0.75)
 	miku_skin.is_unlocked = true
 	skins["miku"] = miku_skin