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]>

debug/dust2_level_builder.gd

@@ -7,7 +7,7 @@ func _build_geometry() -> void:
 	var env = get_node_or_null("WorldEnvironment")
 	if env:
 		var environment = env.environment
-		environment.tonemap_mode = Environment.TONE_MAPPER_ACES
+		# (keep the shared linear tonemap — ACES crushes the cel bands)
 		if environment.sky and environment.sky.sky_material:
 			var sky_mat = environment.sky.sky_material
 			sky_mat.sky_top_color = Color(0.3, 0.5, 0.8)