feat(editor): add --gen-mesh-forge blacksmith hearth

69th procedural mesh primitive. Built from up to 3 axis-
aligned boxes:

  • stone hearth: rectangular base spanning width × depth ×
    baseH, where the smith's fire would sit
  • hood: smaller smoke-collector box on top of the hearth,
    inset by hoodInset on each side so it reads as the
    chimney's flared base
  • optional thin chimney rising from the hood — set
    chimneyH=0 for an open-vented forge

Pairs naturally with --gen-mesh-anvil and
--gen-mesh-workbench for blacksmith/armorer/farrier scenes,
and with --gen-mesh-bench / --gen-mesh-crate-stack for
the storage corner of a smithy.

Watertight under weld (verified 54 manifold edges, 0
boundary, 0 non-manifold). Default 1.4×1.0×0.9 m hearth +
0.5 m hood + 1.2 m chimney reads as a normal village smithy.

tools/editor/cli_arg_required.cpp

@@ -55,7 +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-mesh-outhouse", "--gen-mesh-forge",
     "--gen-camp-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",