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

docs/3D_ASSET_PIPELINE.md

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+# 3D Asset Pipeline for Papaya-Shooter
+
+## Blender Setup
+- Blender 5.0.1 installed system-wide
+- Blender MCP addon at ~/.config/blender/5.0/scripts/addons/blender_mcp/
+- Bridge: ~/blender-mcp/blender_bridge.py (socket port 9876)
+- Start: xvfb-run -a blender --python ~/blender-mcp/blender_server.py &
+
+## Export Workflow
+1. Write Blender Python script (generate model, rig, animate)
+2. Run: blender --background --python script.py
+3. Export as .glb to assets/ subdirectory
+4. Godot auto-imports on editor open
+
+## Directory Structure
+- assets/characters/skins/ — player skin models
+- assets/characters/animations/ — animation clips
+- assets/weapons/models/ — weapon meshes
+- assets/weapons/skins/ — weapon skin textures
+- assets/props/ — level props
+
+## Character System
+- HumanoidModel: characters/humanoid_model.gd
+- Procedural rig: torso, head, 4 arms, 4 legs with pivot nodes
+- States: idle, run, crouch, slide, air, wall_run, wall_cling, grapple, dash, death
+- Color: @export var color (StandardMaterial3D)
+- shadows_only for local player (don't see own model)
+- Weapon attach: set_weapon(script_path) → root_pivot child
+
+## Loadout System  
+- LoadoutManager: globals/loadout_manager.gd (autoload)
+- 5 slots, each: primary_1, primary_2, special, melee
+- Save: user://loadouts.json
+
+## Weapon System
+- WeaponManager: weapons/weapon_manager.gd
+- 1st person: SubViewport + vm_camera (layer 20)
+- 3rd person: HumanoidModel.set_weapon()
+
+## Skin System (NEW)
+- Store skin metadata in loadout or player settings
+- Each skin: display_name, model_path, texture_path, color_tint
+- Apply by replacing HumanoidModel mesh nodes or swapping material
+- For full model replacement: load GLB scene, reparent to player
+
+## Godot 4.2.1 Compatibility
+- absf() → abs()
+- fog_sky_affinity → remove
+- is not Type → not (x is Type)
+- lerpf → lerp
+- := from untyped arrays → explicit type
+- preload() on parse-error scripts → load()