feat(editor): add --gen-texture-snake-skin diamond-scale pattern

60th procedural texture: brick-offset grid of diamond-shaped
scales using L1 (taxicab) metric so |dx|/halfW + |dy|/halfH
< 1 inside each diamond. Even rows are aligned with the cell
grid; odd rows shift by halfW so adjacent scales touch
tangentially along their points — the classic snake/dragon
hide pattern.

A thin dark outline at d ∈ [outlineFrac, 1] gives definition;
outline color is automatically derived as scaleHex × 0.4
(2/5 brightness). Set outlineW=0 to skip the outline for a
smoother painted-scale look.

Distinct from --gen-texture-scales (overlapping circles for
fish/dragon scales) and --gen-texture-chainmail (rings for
metal mail). Useful for snake-cult robe trim, naga skin,
dragon-scale armor inserts, lizardman set dressing,
basilisk-themed dungeon decor.

tools/editor/cli_arg_required.cpp

@@ -92,7 +92,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-starburst", "--gen-texture-studs",
     "--gen-texture-moss", "--gen-texture-woodgrain",
     "--gen-texture-carbon", "--gen-texture-pinstripe",
-    "--gen-texture-camo",
+    "--gen-texture-camo", "--gen-texture-snake-skin",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",