feat(editor): add --gen-mesh-pergola garden-arbor primitive

59th procedural mesh primitive. Builds an open-top pergola
from axis-aligned boxes (uses the new addFlatBox helper):

  • 4 corner posts sized so their outer faces line up with
    the footprint perimeter
  • 2 long perimeter beams resting on top of the posts
    along the long edges
  • N cross beams running between the perimeter beams
    (slightly thinner so the lattice has visible crossings)

Distinct from --gen-mesh-canopy because there's no closed
overhead panel — the open lattice top reads as a sun-trellis
or vine-arbor instead of a market-stall awning. Useful for
palace courtyards, druid groves, garden mazes, viewing
arbors, harvest-festival entrance pieces.

Set crossbeams=0 for a bare 4-post + 2-beam frame; default
5 crossbeams gives a pleasing alternating pattern.

tools/editor/cli_help.cpp

@@ -224,6 +224,8 @@ void printUsage(const char* argv0) {
     std::printf("                         Haystack: terraced stack of N frustums tapering to an apex point (default 0.6/0.9/3/12)\n");
     std::printf("  --gen-mesh-dock <wom-base> [length] [width] [height] [pilings/side] [pilingW] [deckT]\n");
     std::printf("                         Dock: flat plank deck on N pairs of square pilings (default 3.0/1.0/0.6/3/0.10/0.10)\n");
+    std::printf("  --gen-mesh-pergola <wom-base> [length] [width] [height] [postR] [beamT] [crossbeams]\n");
+    std::printf("                         Pergola: 4 corner posts + 2 perimeter beams + N cross beams (open lattice top, no panel)\n");
     std::printf("  --gen-mesh-table <wom-base> [width] [depth] [height] [legThick] [topThick]\n");
     std::printf("                         Table: flat top slab on 4 corner legs (default 1.6/1.0/0.85/0.10/0.06)\n");
     std::printf("  --gen-mesh-lamppost <wom-base> [poleH] [poleT] [baseSize] [lanternSize] [lanternH]\n");