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/test_level_builder.gd

@@ -96,29 +96,9 @@ func _ramp_static(pos: Vector3, size: Vector3, rot_deg: Vector3, color: Color, n
 # ── Environment ───────────────────────────────────────────────────────────────
 
 func _build_environment() -> void:
-	var env := WorldEnvironment.new()
-	env.name = "WorldEnvironment"
-	var environment := Environment.new()
-	environment.background_mode = Environment.BG_SKY
-	var sky := Sky.new()
-	var sky_mat := ProceduralSkyMaterial.new()
-	sky_mat.sky_top_color = Color(0.15, 0.2, 0.35)
-	sky_mat.sky_horizon_color = Color(0.45, 0.5, 0.65)
-	sky_mat.ground_bottom_color = Color(0.1, 0.08, 0.06)
-	sky_mat.ground_horizon_color = Color(0.35, 0.3, 0.25)
-	sky.sky_material = sky_mat
-	environment.sky = sky
-	environment.ambient_light_source = Environment.AMBIENT_SOURCE_SKY
-	environment.ambient_light_energy = 0.4
-	environment.tonemap_mode = Environment.TONE_MAPPER_FILMIC
-	environment.glow_enabled = true
-	environment.glow_intensity = 0.3
-	environment.glow_bloom = 0.1
-	environment.fog_enabled = true
-	environment.fog_light_color = Color(0.5, 0.55, 0.65)
-	environment.fog_density = 0.002
-	env.environment = environment
-	add_child(env)
+	# Shared stylized environment (anime sky, bloom, cel color grade).
+	# Also creates the Sun/FillLight pair since none exists yet.
+	LevelEnvironment.add_to(self)
 
 
 # ── Floor ─────────────────────────────────────────────────────────────────────
@@ -261,24 +241,9 @@ func _build_target_dummy() -> void:
 # ── Lighting ──────────────────────────────────────────────────────────────────
 
 func _build_lighting() -> void:
-	var sun := DirectionalLight3D.new()
-	sun.name = "Sun"
-	sun.rotation_degrees = Vector3(-50, 30, 0)
-	sun.light_color = Color(1.0, 0.95, 0.85)
-	sun.light_energy = 1.2
-	sun.shadow_enabled = true
-	sun.directional_shadow_mode = DirectionalLight3D.SHADOW_PARALLEL_4_SPLITS
-	sun.directional_shadow_max_distance = 100.0
-	add_child(sun)
-
-	# Fill light (opposite side)
-	var fill := DirectionalLight3D.new()
-	fill.name = "FillLight"
-	fill.rotation_degrees = Vector3(-30, -150, 0)
-	fill.light_color = Color(0.6, 0.7, 0.9)
-	fill.light_energy = 0.3
-	fill.shadow_enabled = false
-	add_child(fill)
+	# Sun and fill are created by LevelEnvironment.add_to in _build_environment;
+	# kept as a hook for subclasses that re-style them (see dust2).
+	pass
 
 
 # ── Player ────────────────────────────────────────────────────────────────────