feat: cel-shaded look — toon shader, ink outlines, stylized environment

- toon.gdshader: banded diffuse with cool-tinted shadows (3 tones), stepped
  specular, rim light, and an optional world-triplanar albedo path so level
  geometry keeps the 0.5 m grid with no UVs
- toon_outline.gdshader: inverted-hull ink outline, applied via
  material_overlay so it works on skinned (deforming) meshes
- LevelMaterials: tinted() now returns toon grid materials; toonify()/
  apply_toon_recursive() convert imported character/prop materials in place
- SkinnedPlayerModel: characters get toon shading + outline on load
- LevelEnvironment: shared stylized environment for all maps — saturated
  anime sky, linear tonemap (filmic/ACES crush cel bands), bloom for
  emissives, saturation+contrast grade, light depth haze; test level and
  dust2 now use it and the procedural arena (which had NO environment)
  gets it plus runtime toonification of its baked geometry

Co-Authored-By: Claude Fable 5 <[email protected]>

characters/skinned_player_model.gd

@@ -108,6 +108,9 @@ func load_model(path: String) -> void:
 		_pose_mod = ShooterPoseModifier.new()
 		_pose_mod.name = "ShooterPose"
 		skeleton.add_child(_pose_mod)
+
+	# Cel-shaded look: toon shading over the imported textures + ink outline.
+	LevelMaterials.apply_toon_recursive(scene)
 	if animation_player:
 		_index_animations()
 	else: