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_arg_required.cpp

@@ -57,6 +57,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-training-dummy", "--gen-mesh-hitching-post",
     "--gen-mesh-outhouse", "--gen-mesh-forge",
     "--gen-mesh-archery-target", "--gen-mesh-gravel-pile",
+    "--gen-mesh-stone-bench",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",