feat(editor): add --gen-mesh-tombstone graveyard primitive

47th procedural mesh: 3-box vertical headstone — wide low
base plinth, tall thin main slab on top, and a slightly-
wider decorative crown that acts as a flat-cap stand-in for
the arched-top headstone shape (which would need rotated
faces). Pairs with --gen-mesh-grave (horizontal mound) and
--gen-mesh-coffin for graveyards, dungeon crypts, haunted-
zone set dressing.

Vertical layout: 15% base, 75% slab, 10% crown. Defaults to
0.60 wide x 1.10 tall x 0.18 deep with the base 1.45x wider
than the slab on each footprint axis.

tools/editor/main.cpp

@@ -145,7 +145,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-throne", "--gen-mesh-coffin", "--gen-mesh-bookshelf",
         "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
         "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",
-        "--gen-mesh-mailbox",
+        "--gen-mesh-mailbox", "--gen-mesh-tombstone",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",