feat(editor): add --gen-texture-caustics water-shimmer pattern

52nd procedural texture: water-surface caustics via four
superimposed sine waves running along x, y, x+y, and x-y.
Each wave is taken in absolute value before multiplication
so peaks shine on either side of the wave centerline,
giving the bright-line interference network you see on a
pool floor or sunlit shallow streambed.

Two-color lerp from bgHex (depth) to hiHex (highlight).
Default 24-pixel period reads at the right scale for water-
plane underlay; pass a smaller period for tighter ripple.

Useful as the floor texture under transparent water
volumes, magic-fountain pools, fish-tank ground plates,
ritual scrying basins, and any "submerged surface seen
through clear water" effect.

tools/editor/cli_arg_required.cpp

@@ -85,7 +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",
+    "--gen-texture-rope", "--gen-texture-caustics",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",