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

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

characters/skin_manager.gd

@@ -0,0 +1,62 @@
+extends Node
+class_name SkinManager
+
+## Manages all available player skins.
+## Add new skins here or load from a config file.
+
+var skins: Dictionary = {}
+var active_skin_id: String = "default"
+
+func _ready() -> void:
+	_register_default_skins()
+
+func _register_default_skins() -> void:
+	# Default blue soldier
+	var default_skin = PlayerSkin.new()
+	default_skin.skin_name = "Default"
+	default_skin.description = "Standard issue"
+	default_skin.color_tint = Color(0.2, 0.4, 0.8)
+	default_skin.is_unlocked = true
+	skins["default"] = default_skin
+
+	# Red team
+	var red_skin = PlayerSkin.new()
+	red_skin.skin_name = "Red Team"
+	red_skin.description = "Red team colors"
+	red_skin.color_tint = Color(0.8, 0.2, 0.2)
+	red_skin.is_unlocked = true
+	skins["red_team"] = red_skin
+
+	# Green soldier
+	var green_skin = PlayerSkin.new()
+	green_skin.skin_name = "Forest"
+	green_skin.description = "Forest camouflage"
+	green_skin.color_tint = Color(0.2, 0.6, 0.2)
+	green_skin.is_unlocked = true
+	skins["forest"] = green_skin
+
+	# Hatsune Miku
+	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.color_tint = Color(0.0, 0.75, 0.75)
+	miku_skin.is_unlocked = true
+	skins["miku"] = miku_skin
+
+	print("SkinManager: registered %d skins" % skins.size())
+	for id in skins:
+		print("  - %s: %s" % [id, skins[id].skin_name])
+
+func get_skin(skin_id: String) -> PlayerSkin:
+	if skins.has(skin_id):
+		return skins[skin_id]
+	return skins["default"]
+
+func get_active_skin() -> PlayerSkin:
+	return get_skin(active_skin_id)
+
+func set_active_skin(skin_id: String) -> void:
+	if skins.has(skin_id) and skins[skin_id].is_unlocked:
+		active_skin_id = skin_id
+		print("SkinManager: active skin set to '%s'" % skins[skin_id].skin_name)