feat(editor): add --gen-texture-diamond-grid solid-shape tile

66th procedural texture: axis-aligned grid of solid diamond
shapes (no row offset) separated by visible bg gaps. Each
cell hosts one diamond computed via L1 (taxicab) metric:
|dx|/halfW + |dy|/halfH < fillFrac.

Distinct from --gen-texture-snake-skin (brick-offset diamonds
that touch tangentially with a derived dark outline) — this
variant uses uniform spacing and a configurable fill fraction
so diamonds float in clean grid rows.

fillFrac = 0.80 default gives a 20% bg-gap moat between
diamonds; bump near 1.0 for nearly-touching chevron-floor
look, drop to 0.5 for sparse polka-diamond.

Useful for clean tile floors, mosaic inlay panels, banner
seamless backgrounds, formal heraldry trim, jeweled-cloak
tabards.

tools/editor/cli_arg_required.cpp

@@ -99,7 +99,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-camo", "--gen-texture-snake-skin",
     "--gen-texture-mesh-screen", "--gen-texture-bamboo",
     "--gen-texture-blueprint", "--gen-texture-rust-streaks",
-    "--gen-texture-plaid",
+    "--gen-texture-plaid", "--gen-texture-diamond-grid",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",