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]>

assets/shaders/toon_outline.gdshader

@@ -0,0 +1,17 @@
+shader_type spatial;
+render_mode cull_front, unshaded;
+
+// Inverted-hull outline. Assign as material_overlay on a GeometryInstance3D:
+// the mesh renders a second time, grown along its normals with front faces
+// culled, leaving a colored shell visible only at the silhouette.
+
+uniform vec4 outline_color : source_color = vec4(0.06, 0.05, 0.09, 1.0);
+uniform float outline_width : hint_range(0.0, 0.1) = 0.02;
+
+void vertex() {
+	VERTEX += NORMAL * outline_width;
+}
+
+void fragment() {
+	ALBEDO = outline_color.rgb;
+}