feat(editor): add --gen-mesh-outhouse small-shed primitive

68th procedural mesh primitive. Built from 3 axis-aligned
boxes:

  • solid body slab spanning width × depth × (height-roofT)
  • thin door slab on the +Z face (pushed slightly outward
    so it visually sits on the wall — gives doorframe-like
    relief without a real cutout)
  • roof slab slightly larger than the body footprint and
    sitting on top, controlled by roofOverhang

Distinct from --gen-mesh-house (multi-walled, peaked-roof
dwelling). An outhouse is the single-room privy / tool shed
variant — useful for villages, frontier outposts, small
farms, refuse-shed corners.

Watertight under weld (verified 54 manifold edges, 0
boundary, 0 non-manifold). First handler to use the new
setCenteredBoundsXZ helper from this batch.

tools/editor/cli_arg_required.cpp

@@ -55,6 +55,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-workbench", "--gen-mesh-crate-stack",
     "--gen-mesh-watchpost", "--gen-mesh-water-trough",
     "--gen-mesh-training-dummy", "--gen-mesh-hitching-post",
+    "--gen-mesh-outhouse",
     "--gen-camp-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",