feat(editor): add --gen-texture-moss organic-mottling pattern

55th procedural texture: irregular spots scattered on a hash-
jittered grid. Each cell tests its 8 neighbors so spots near
cell boundaries don't get clipped at the cell wall.

Per-cell hashing controls:
  • presence — density 0-100 chance the cell hosts a spot
  • position — uniform jitter inside the cell
  • radius — 25-75 % of stride, so adjacent spots vary in size

Result reads as random patches of organic growth rather than
a visible lattice. Useful for forest floors, weathered stone
walls, dungeon flagstones, swamp ground, ruined-temple
overlay, fungal-cavern detail.

Default 16-stride / 70-density on a stone-grey base reads
cleanly at 256x256 with about half the surface mossed over.

tools/editor/cli_arg_required.cpp

@@ -88,6 +88,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-planks", "--gen-texture-corrugated",
     "--gen-texture-rope", "--gen-texture-caustics",
     "--gen-texture-starburst", "--gen-texture-studs",
+    "--gen-texture-moss",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",