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/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)