feat(editor): add --gen-mesh-stone-bench heavy seating

72nd procedural mesh primitive. Long stone bench from 3
axis-aligned boxes:

  • horizontal seat slab spanning the full length on top
  • 2 vertical block supports near the ends, inset by
    supportInset from each end so the seat slab overhangs
    them slightly (the standard "seat rests on legs" look)

Distinct from --gen-mesh-bench (wooden 4-leg construction
with thinner pieces) — this is the heavier stone variant
for parks, temple courtyards, ruined cities, dwarven mead
halls, library-courtyard reading nooks.

Watertight under weld (verified 54 manifold edges, 0
boundary, 0 non-manifold). Default 2.0 m × 0.4 m bench at
0.45 m sit height reads at adult-human proportions.

Uses every shared helper added in recent batches —
new gen-mesh handlers now consistently land at ~50 lines
including parse + validate + geometry + finalization.

tools/editor/cli_help.cpp

@@ -276,6 +276,8 @@ void printUsage(const char* argv0) {
     std::printf("                         Archery target: round face cylinder on a 2-post stand with cross-beam (training yard / fair scene)\n");
     std::printf("  --gen-mesh-gravel-pile <wom-base> [stoneCount] [baseR] [pileH] [maxStoneSize] [seed]\n");
     std::printf("                         Gravel pile: hash-distributed stone cubes in a conical heap (mining / construction / rubble)\n");
+    std::printf("  --gen-mesh-stone-bench <wom-base> [length] [depth] [seatH] [seatT] [supportW] [supportInset]\n");
+    std::printf("                         Stone bench: long seat slab on 2 block supports near the ends (park / temple / ruined city)\n");
     std::printf("  --gen-camp-pack <outDir>\n");
     std::printf("                         Convenience: emit tent + firepit + bedroll + canopy + woodpile + haystack into outDir as 6 .wom files\n");
     std::printf("  --gen-blacksmith-pack <outDir>\n");