feat(editor): add --gen-texture-halftone gradient-modulated dots

80th procedural texture: regular grid of dots whose radii
grow with a configurable gradient direction. Three modes:

  • v (vertical)    — radii grow top-to-bottom
  • h (horizontal)  — radii grow left-to-right
  • r (radial)      — radii grow from texture center outward

Mimics the comic-print / newspaper image-reproduction trick
of varying dot size to encode grayscale. Distinct from
--gen-texture-dots (uniform radius across the grid) and
--gen-texture-studs (uniform with derived inner highlight)
— halftone is the gradient-modulated variant.

Useful for retro-comic / newspaper-aesthetic surfaces, vintage-
poster overlays, sci-fi monitor screens (radial mode reads as
CRT vignette), banner gradient detail, dawn/dusk sky overlays.

Default 16-stride / maxR=7 / vertical reads as classic comic
shading at 256x256.

tools/editor/cli_arg_required.cpp

@@ -119,7 +119,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-embroidery", "--gen-texture-lightbeam",
     "--gen-texture-dewdrops", "--gen-texture-pinwheel",
     "--gen-texture-scratched-metal", "--gen-texture-crackle",
-    "--gen-texture-star",
+    "--gen-texture-star", "--gen-texture-halftone",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",