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/humanoid_model.gd

@@ -163,6 +163,49 @@ func _ready() -> void:
 	calf_r.cast_shadow = shadow_setting
 	calf_r_pivot.add_child(calf_r)
 
+## Skin System
+var current_skin: PlayerSkin
+var skin_model: Node3D
+
+func apply_skin(skin: PlayerSkin) -> void:
+	current_skin = skin
+	if skin.model_path != "" and skin.model_path != null:
+		var loaded = load(skin.model_path)
+		if loaded:
+			_set_procedural_visible(false)
+			if skin_model:
+				skin_model.queue_free()
+				skin_model = null
+			skin_model = loaded.instantiate()
+			if skin_model:
+				skin_model.name = "SkinModel"
+				add_child(skin_model)
+				print("HumanoidModel: applied skin '%s'" % skin.skin_name)
+			return
+	_set_procedural_visible(true)
+	if skin_model:
+		skin_model.queue_free()
+		skin_model = null
+	_apply_color(skin.color_tint)
+
+func _set_procedural_visible(visible: bool) -> void:
+	for part in [torso, head, upper_arm_l, lower_arm_l, upper_arm_r, lower_arm_r, thigh_l, calf_l, thigh_r, calf_r]:
+		if part:
+			part.visible = visible
+
+func _apply_color(col: Color) -> void:
+	var mat = StandardMaterial3D.new()
+	mat.albedo_color = col
+	mat.roughness = 0.8
+	for part in [torso, upper_arm_l, lower_arm_l, upper_arm_r, lower_arm_r, thigh_l, calf_l, thigh_r, calf_r]:
+		if part and part.mesh:
+			part.mesh.material = mat
+	if head and head.mesh:
+		var hmat = StandardMaterial3D.new()
+		hmat.albedo_color = Color(0.95, 0.85, 0.78)
+		hmat.roughness = 0.8
+		head.mesh.material = hmat
+
 func update_state(state: String, speed: float, is_crouching: bool = false) -> void:
 	current_state = state
 	movement_speed = speed