feat(editor): add --gen-texture-moon disc with phase shadow

82nd procedural texture: solid filled circle of moonHex on
bg, with an optional crescent shadow created by subtracting
a second bg-colored disc offset by `phase` pixels along +X.

Phase reads as the lunar-cycle moment:
  • phase = 0           — full moon
  • 0 < phase < moonR    — gibbous (waning right)
  • phase = moonR        — half moon
  • phase > moonR        — crescent (thinner as phase grows)

Useful for night-sky overlays, banner moons (heraldry of
night-themed orders), shrine medallion centers, druid /
priest emblem inserts, lunar-festival decoration, vampire-
zone backdrops.

The simplest centered-disc texture in the catalogue —
pairs naturally with --gen-texture-stars for a complete
night-sky composite.

tools/editor/cli_arg_required.cpp

@@ -125,7 +125,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-dewdrops", "--gen-texture-pinwheel",
     "--gen-texture-scratched-metal", "--gen-texture-crackle",
     "--gen-texture-star", "--gen-texture-halftone",
-    "--gen-texture-bayer",
+    "--gen-texture-bayer", "--gen-texture-moon",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",