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.gdshader

@@ -0,0 +1,80 @@
+shader_type spatial;
+
+// Cel/toon surface shader used across the game.
+//  - Banded (stepped) diffuse with a tinted shadow color instead of black
+//  - Soft rim light for silhouette pop
+//  - Optional world-space triplanar albedo (level geometry: no UVs needed,
+//    grid stays world-scaled) — otherwise standard UV sampling (characters)
+
+uniform vec4 albedo_color : source_color = vec4(1.0);
+uniform sampler2D albedo_texture : source_color, filter_linear_mipmap, repeat_enable;
+uniform bool has_texture = true;
+uniform bool use_triplanar = false;
+uniform float triplanar_tile = 2.0;      // world units per texture tile
+
+uniform float band_edge : hint_range(-1.0, 1.0) = 0.05;   // NdotL where light band starts
+uniform float band_softness : hint_range(0.001, 0.5) = 0.04;
+uniform float mid_band_edge : hint_range(-1.0, 1.0) = 0.55; // second, brighter band
+uniform vec4 shadow_color : source_color = vec4(0.62, 0.65, 0.78, 1.0); // cool shadow tint
+uniform float rim_strength : hint_range(0.0, 2.0) = 0.35;
+uniform float rim_width : hint_range(0.0, 1.0) = 0.65;
+uniform float specular_strength : hint_range(0.0, 1.0) = 0.25;
+uniform float specular_shininess : hint_range(1.0, 128.0) = 24.0;
+
+varying vec3 world_pos;
+varying vec3 world_normal;
+
+void vertex() {
+	world_pos = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xyz;
+	world_normal = normalize((MODEL_MATRIX * vec4(NORMAL, 0.0)).xyz);
+}
+
+vec3 sample_triplanar(vec3 p, vec3 n) {
+	vec3 w = abs(n);
+	w = pow(w, vec3(4.0));
+	w /= (w.x + w.y + w.z);
+	vec2 uv_x = p.zy / triplanar_tile;
+	vec2 uv_y = p.xz / triplanar_tile;
+	vec2 uv_z = p.xy / triplanar_tile;
+	vec3 cx = texture(albedo_texture, uv_x).rgb;
+	vec3 cy = texture(albedo_texture, uv_y).rgb;
+	vec3 cz = texture(albedo_texture, uv_z).rgb;
+	return cx * w.x + cy * w.y + cz * w.z;
+}
+
+void fragment() {
+	vec3 base = albedo_color.rgb;
+	if (has_texture) {
+		if (use_triplanar) {
+			base *= sample_triplanar(world_pos, world_normal);
+		} else {
+			base *= texture(albedo_texture, UV).rgb;
+		}
+	}
+	ALBEDO = base;
+	ROUGHNESS = 1.0;
+	SPECULAR = 0.0;
+
+	// Rim: brighten grazing angles for that inked-silhouette pop.
+	float rim = 1.0 - clamp(dot(normalize(VIEW), NORMAL), 0.0, 1.0);
+	rim = smoothstep(1.0 - rim_width, 1.0, rim);
+	EMISSION = base * rim * rim_strength;
+}
+
+void light() {
+	float ndotl = dot(NORMAL, LIGHT);
+	// Shadowing folds into the band test so shadow edges band too.
+	float lit = ndotl * ATTENUATION;
+	float band = smoothstep(band_edge - band_softness, band_edge + band_softness, lit);
+	float mid = smoothstep(mid_band_edge - band_softness, mid_band_edge + band_softness, lit);
+	// 3 tones: shadow tint -> base band (0.82) -> full light.
+	float tone = mix(0.82, 1.0, mid);
+	vec3 shade = mix(shadow_color.rgb, vec3(tone), band);
+	DIFFUSE_LIGHT += ALBEDO * LIGHT_COLOR / PI * shade;
+
+	// Stepped specular dot for glossy toon highlights.
+	vec3 h = normalize(VIEW + LIGHT);
+	float spec = pow(clamp(dot(NORMAL, h), 0.0, 1.0), specular_shininess);
+	spec = smoothstep(0.5 - band_softness, 0.5 + band_softness, spec);
+	SPECULAR_LIGHT += LIGHT_COLOR * spec * specular_strength * band;
+}