feat(editor): add --gen-mesh-ladder utility primitive

43rd procedural mesh: 2 vertical rails + N evenly-spaced
horizontal rungs. Sits flat against +Z (the climbing face)
so it can be parented to walls, wagons, ship hulls, mage
tower trapdoors, attic openings, dungeon shafts.

Defaults to 3.0m tall × 0.6m wide with 8 rungs (typical
home/loft ladder). Rung spacing computes as height/(rungs+1)
so the first rung sits a half-step from the floor and the
last rung the same distance from the top — keeps the ladder
visually symmetric regardless of rung count.

tools/editor/main.cpp

@@ -137,6 +137,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-shrine", "--gen-mesh-totem", "--gen-mesh-cage",
         "--gen-mesh-throne", "--gen-mesh-coffin", "--gen-mesh-bookshelf",
         "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
+        "--gen-mesh-ladder",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",