feat(skin): add skin system + Miku character skin

Skin System:
- PlayerSkin resource class (model_path, color_tint, etc.)
- SkinManager with default skins: Default, Red Team, Forest, Miku
- HumanoidModel.apply_skin() supports both GLB model replacement
  and procedural color tinting
- Procedural meshes auto-hide when GLB model is loaded

Miku Skin:
- 43-part procedural model generated in Blender (headless)
- Twin tails, headphones, thigh-high socks, idol outfit
- Teal/cyan color scheme with red accents
- Exported to assets/characters/skins/miku.glb
- Applied to player 1 in test level builder

Pipeline:
- Blender 5.0.1 + MCP addon installed
- Bridge script for socket-based Blender control
- docs/3D_ASSET_PIPELINE.md documents full workflow

debug/test_level_builder.gd

@@ -377,6 +377,14 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 	humanoid.position = Vector3(0, -0.9, 0)
 	player.add_child(humanoid)
 
+	# Apply skin (Miku for player 1, default for others)
+	if pid == 1:
+		var miku_skin = PlayerSkin.new()
+		miku_skin.skin_name = "Miku"
+		miku_skin.model_path = "res://assets/characters/skins/miku.glb"
+		miku_skin.color_tint = Color(0.0, 0.75, 0.75)
+		humanoid.apply_skin(miku_skin)
+
 	# Movement State Machine
 	var sm := Node.new()
 	sm.name = "MovementStateMachine"