feat(editor): add --gen-texture-mold Worley-noise patches

72nd procedural texture: Worley (cellular) noise thresholded
into mold patches. Each grid cell hosts a hash-jittered
center; each pixel computes its distance to the nearest
center across 9 surrounding cells (current + 8 neighbors)
and is painted as mold if that distance is below
thresholdFrac × stride.

Distinct from --gen-texture-moss (single-spot per cell with
hash-derived presence/jitter/radius) — mold has irregular
field-shaped patches following the Voronoi cell boundaries
rather than discrete circles, mimicking real fungal growth
along surfaces.

Useful for cellars, dungeon walls, plague zones, sewer
overflow, food-warehouse spoilage, witch-hut detail trim.
Default 18-stride / 0.55-threshold gives a moderate
infestation; bump threshold near 0.9 for full coverage.

tools/editor/cli_arg_required.cpp

@@ -107,7 +107,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-plaid", "--gen-texture-diamond-grid",
     "--gen-texture-houndstooth", "--gen-texture-chevron",
     "--gen-texture-dunes", "--gen-texture-swirl",
-    "--gen-texture-ironbark",
+    "--gen-texture-ironbark", "--gen-texture-mold",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",