feat(editor): add --gen-mesh-mine-cart underground transport

73rd procedural mesh primitive. Open-top mining cart on
4 wheel boxes:

  • bin: 5-piece basin construction (bottom slab + 4
    perimeter walls), same arrangement as
    --gen-mesh-water-trough
  • wheels: 4 cube boxes at the corners, inset by
    wheelInset and sitting on the ground (y from 0 to
    2*wheelR)
  • bin sits on top of the wheels at y = 2*wheelR

All axis-aligned — exercises every shared helper. Useful
for mines, dwarven forges, gnomish junk-yards, abandoned-
tunnel set dressing, salvage scenes.

Default 0.9 × 0.5 m bin with 0.08-radius wheels gives the
classic narrow-gauge ore-cart silhouette. The 4-wall basin
construction (verified via --info-mesh-stats: 6 non-manifold
edges at the wall corners — same as --gen-mesh-water-trough's
expected branchy edges) means the cart can hold collision-
visible ore inside if a content author drops in another
primitive.

tools/editor/cli_arg_required.cpp

@@ -57,7 +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-mesh-stone-bench", "--gen-mesh-mine-cart",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",