feat(editor): add --gen-texture-cracked branching-walk pattern

43rd procedural texture: organic crack network done via
recursive random walks from N seed nuclei. Each seed spawns
a crack that walks in a random direction for some length,
then with 60% chance branches into one or two more cracks
of half-remaining length. Most cracks die out after a step
or two, a few branch into longer networks — the bias matches
real-world fissure formation.

Useful for cracked mud, dry earth, broken glass, weathered
stone, dragon skin overlays, ice-shard effects. Defaults
to 12 seeds at 40-px max length. Iterative DFS instead of
true recursion so deep branching chains never blow the
stack.

tools/editor/main.cpp

@@ -179,6 +179,7 @@ int main(int argc, char* argv[]) {
         "--gen-texture-parquet", "--gen-texture-bubbles",
         "--gen-texture-spider-web", "--gen-texture-gingham",
         "--gen-texture-lattice", "--gen-texture-honeycomb",
+        "--gen-texture-cracked",
         "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
         "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
         "--validate-png", "--validate-blp",