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

characters/player_skin.gd

@@ -0,0 +1,12 @@
+extends Resource
+class_name PlayerSkin
+
+## A player skin defines the visual appearance of the character.
+## Can be either a color tint (for the default procedural model)
+## or a full GLB model replacement.
+
+@export var skin_name: String = "Default"
+@export var description: String = ""
+@export var model_path: String = ""  # Path to .glb file, empty = use procedural
+@export var color_tint: Color = Color(0.2, 0.4, 0.8)  # For procedural model
+@export var is_unlocked: bool = true