feat(editor): add --gen-texture-dunes wave-ripple pattern

69th procedural texture: stack of parallel sinusoidal curves
spaced verticalSpacing apart. A pixel falls on a dune line
if its Y is within lineW/2 pixels of the nearest curve at
y = N*spacing + amp*sin(2π*x/period).

Distinct from --gen-texture-corrugated (uniform parallel
lines) and --gen-texture-zebra (sin-shifted strip fills) —
this is the discrete-curve variant for desert ground
textures and shallow-water sand patterns.

Useful for desert ground, shallow-water sand bottoms,
beach close-ups, mirage scene-setters, sand-storm overlays,
zone-specific weathering on stone surfaces.

tools/editor/cli_arg_required.cpp

@@ -105,6 +105,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-blueprint", "--gen-texture-rust-streaks",
     "--gen-texture-plaid", "--gen-texture-diamond-grid",
     "--gen-texture-houndstooth", "--gen-texture-chevron",
+    "--gen-texture-dunes",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",