feat(editor): add --gen-texture-ironbark hardwood-plate pattern

71st procedural texture: vertical wood-grain streaks (like
--gen-texture-bark) overlaid with horizontal "plate" bands
at regular Y intervals — the segmented look of mature
hardwood / ironwood / sycamore bark.

Per-pixel logic:
  • horizontal crack: top crackW pixels of every plateY
    cycle gets the dark crack color
  • vertical streak: per-streak hash determines an offset
    crack position within each streakSpacing block
  • elsewhere: base color modulated by per-streak-and-plate
    brightness jitter (±15) so the surface has natural
    texture variation

Distinct from --gen-texture-bark (vertical-crack only) and
--gen-texture-wood (vertical-streak only). Useful for ancient
trees, druid grove deadwood, dwarven hardwood beams,
fortress door planks, totem-pole carvings.

tools/editor/cli_arg_required.cpp

@@ -107,6 +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",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",