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]>

scenes/maps/procedural_arena/procedural_arena_runtime.gd

@@ -18,7 +18,17 @@ var _debug_ui_panel: PanelContainer
 func _ready() -> void:
 	# Hide mouse
 	Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
-	
+
+	# Cel-shaded presentation: stylized sky/grade + toon materials over the
+	# arena's baked geometry. The scene ships its own DirectionalLight3D;
+	# rename it to Sun so the helper doesn't stack extra suns.
+	var existing_light := get_node_or_null("DirectionalLight3D")
+	if existing_light:
+		existing_light.name = "Sun"
+	if not has_node("WorldEnvironment"):
+		LevelEnvironment.add_to(self)
+	LevelMaterials.apply_toon_recursive(self, 0.0)
+
 	_build_hud()
 	
 	# Multiplayer Spawning