feat(editor): add --gen-texture-dewdrops scattered-droplets

75th procedural texture: N small water droplets at hash-
derived (cx, cy, R) positions blended onto bg via radial
brightness — bright at the drop center fading to bg at its
edge. Where drops overlap they accumulate brighter
(max-of-individual-contributions, not additive — so the
result stays inside the [bg, drop] color range).

Each drop gets a hash-derived radius in the [maxR/4, maxR]
range so the surface has a natural mix of small and large
droplets. Two-pass (compute brightness map, then write
pixels) so overlap accumulation works correctly.

Useful for morning-grass blades, wet-glass overlays, leaf
surfaces after rain, magic-pool fizz, snake-fountain
splash zones, dragon-breath-aftermath ground.

tools/editor/cli_arg_required.cpp

@@ -110,6 +110,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-dunes", "--gen-texture-swirl",
     "--gen-texture-ironbark", "--gen-texture-mold",
     "--gen-texture-embroidery", "--gen-texture-lightbeam",
+    "--gen-texture-dewdrops",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",