feat: full cel treatment for Neon Alley — flat toon colors + ink outlines

- LevelMaterials.flat(): toon-banded FLAT color material (no greybox grid
  texture) for dressed maps; cached per tint, rim/spec off
- Neon Alley overrides _box_static: every structure renders flat cel color;
  prop-scale pieces (<10m) get an inverted-hull ink outline scaled to their
  size. Building-scale boxes skip the hull — the inflated plates smear into
  black voids at close range (found via fixed street-camera capture)
- Pylon wires render as flat ink lines; warmer wood + hotter vermilion for
  flat shading (the grid multiply used to brighten tints)
- visual_capture: deterministic street-level shot after the aerial for
  style comparisons

Street capture now reads properly BoomerangX: flat lavender asphalt, hard
light bands, torii framed down the underpass, neon strips + sign glow.

Co-Authored-By: Claude Fable 5 <[email protected]>

scenes/maps/level_materials.gd

@@ -51,6 +51,26 @@ static func tinted(tint: Color, dark: bool = false) -> Material:
 	return mat
 
 
+static var _flat_cache: Dictionary = {}
+
+## Flat cel color: toon banding with NO grid texture — the full stylized
+## look for dressed maps (vs. tinted()'s greybox grid for blockouts).
+## Rim/specular stay off (they blob on large level surfaces).
+static func flat(tint: Color) -> Material:
+	var key := tint.to_html()
+	if _flat_cache.has(key):
+		return _flat_cache[key]
+	var mat := ShaderMaterial.new()
+	mat.shader = load(TOON_SHADER)
+	mat.set_shader_parameter("has_texture", false)
+	mat.set_shader_parameter("use_triplanar", false)
+	mat.set_shader_parameter("albedo_color", tint)
+	mat.set_shader_parameter("rim_strength", 0.0)
+	mat.set_shader_parameter("specular_strength", 0.0)
+	_flat_cache[key] = mat
+	return mat
+
+
 ## Toon version of an arbitrary material (usually a character's imported
 ## StandardMaterial3D): keeps its albedo texture/color, swaps the shading.
 static func toonify(src: Material) -> Material: