feat(editor): add --gen-mesh-gate wooden farm-gate primitive

51st procedural mesh: 5-box wooden gate — 2 vertical posts on
either side and 3 horizontal cross rails (top, middle, bottom)
spanning the opening. The gate sits flat in the X-Y plane
(rails along X, posts along Y) so it can hang in a wall slot
or fence gap without needing rotation.

Useful for fenced fields, manor entrances, pen openings,
courtyard barriers, dungeon archway closures. Defaults to a
1.80m opening with 1.30m-tall posts. Pairs with
--gen-mesh-fence for complete enclosures, exporting through
the open-format ecosystem (WOM -> OBJ / GLB / STL) so designers
can preview the gate-to-fence joinery in any external tool.

tools/editor/cli_help.cpp

@@ -212,6 +212,8 @@ void printUsage(const char* argv0) {
     std::printf("                         Stool: small backless seat on 4 corner legs (default 0.36/0.04/0.45/0.04)\n");
     std::printf("  --gen-mesh-cauldron <wom-base> [rimWidth] [bodyHeight] [legHeight]\n");
     std::printf("                         Cauldron: 4 legs + narrow bottom + wider mid + widest rim tiers (default 0.80/0.70/0.10)\n");
+    std::printf("  --gen-mesh-gate <wom-base> [openingWidth] [postHeight] [postT] [railT]\n");
+    std::printf("                         Gate: 2 vertical posts + 3 horizontal rails (default 1.80/1.30/0.10/0.06)\n");
     std::printf("                         Procedural tree: cylindrical trunk + spherical foliage (default 0.1/2.0/0.7)\n");
     std::printf("  --displace-mesh <wom-base> <heightmap.png> [scale]\n");
     std::printf("                         Offset each vertex along its normal by heightmap brightness × scale (default 1.0)\n");