feat(editor): add --gen-mesh-stool small-furniture primitive

49th procedural mesh: 5-box backless stool — flat square seat
on 4 short legs at the corners. Smaller-footprint counterpart
to --gen-mesh-bench, pairs with --gen-mesh-table for taverns,
workshops, NPC market stalls, dwarven taprooms.

Defaults to 0.36m square seat at 0.49m total height — typical
3-legged-bar-stool proportions. Customizable per dimension so
the same primitive covers child stools, milking stools, and
tall workshop stools.

tools/editor/main.cpp

@@ -150,6 +150,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
         "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",
         "--gen-mesh-mailbox", "--gen-mesh-tombstone", "--gen-mesh-crate",
+        "--gen-mesh-stool",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",