feat(editor): add --gen-texture-rope twisted-cordage pattern

51st procedural texture: two interleaved sinusoidal strands
running along the Y axis. Each strand's X position oscillates
as W/4·sin(2π·y/period); the second strand is phase-shifted
by π so the two snake around each other forming the classic
helical twist of a tightened rope.

Within each strand, brightness rises from 0.55 at the edge
to 1.0 at the centerline (cos² falloff) — gives the rounded
3D appearance of cylindrical fibers without a separate
shadow pass.

Useful for hanging ropes, ship rigging, tied-bundle textures,
suspension bridges, market-stall awning ties, and any
"twisted cord" surface where a flat color would read as
ribbon rather than rope.

Default 24-pixel period with 8-pixel strands renders cleanly
at 256x256 with about 10 visible twists.

tools/editor/cli_arg_required.cpp

@@ -85,6 +85,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-leopard", "--gen-texture-zebra",
     "--gen-texture-knit", "--gen-texture-chainmail",
     "--gen-texture-planks", "--gen-texture-corrugated",
+    "--gen-texture-rope",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",