feat(editor): add --gen-texture-embroidery cross-stitch grid

73rd procedural texture: a grid of X-shape cross stitches.
Each cell holds an X formed by two diagonal strokes from
opposing cell corners through the center. A pixel falls on
a stitch if its in-cell position is within strokeW/2 of
either diagonal axis.

Distinct from --gen-texture-checker (filled squares),
--gen-texture-knit (V-stitch chevron), --gen-texture-lattice
(diagonal trellis). Embroidery is the explicit two-direction
diagonal stitch mark used by counted-thread textile work.

Useful for sampler-cloth detail, folk-art trim, peasant-
clothier border bands, witch-hut wall hangings, hearth-rug
overlays. Default 12-cell / 2-stroke gives a fine X-stitch
look; bump cellSize for chunkier sampler stitches.

tools/editor/cli_arg_required.cpp

@@ -109,6 +109,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-houndstooth", "--gen-texture-chevron",
     "--gen-texture-dunes", "--gen-texture-swirl",
     "--gen-texture-ironbark", "--gen-texture-mold",
+    "--gen-texture-embroidery",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",