feat(editor): add --gen-mesh-mailbox wayside primitive

46th procedural mesh: 4-box mailbox — vertical post with a
horizontal box body on top and a small flag (pole + plate)
mounted on the right (+X) side near the front. Useful for
inns, post stations, manor gates, frontier outposts, courier
routes between zones.

Defaults to a 1.10m post + 0.45L x 0.20W x 0.20H body
(~1.44m total). The box body is intentionally slightly wider
than the post on each axis so the body visually caps the
post. Flag plate extends +X away from the body so it reads
as a raised flag from the road side.

tools/editor/main.cpp

@@ -143,6 +143,7 @@ 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-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",