feat(editor): add --gen-mesh-chimney brick-stack primitive

60th procedural mesh primitive. Builds a chimney as two
axis-aligned boxes (uses the shared addFlatBox helper):

  • main shaft: rectangular brick stack of width × depth ×
    (height - capH)
  • protective cap: a slightly wider box on top (capExtra
    wider than shaft on every side) — the rain-throw crown
    that sits on real masonry chimneys

Set capH=0 for a bare unfinished stack. Default 0.45×0.45×
1.8 m shaft with a 0.10-tall cap reads at human scale for
rooftop placement.

Useful for cottage rooflines, blacksmith forges, alchemist
labs, dwarven longhouses — any structure with a fireplace
that needs visible vent. Watertight under weld (verified
via --info-mesh-stats: 36 manifold edges, 0 boundary, 0
non-manifold).

tools/editor/cli_arg_required.cpp

@@ -51,7 +51,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-throne", "--gen-mesh-coffin", "--gen-mesh-bookshelf",
     "--gen-mesh-tent", "--gen-mesh-firepit", "--gen-mesh-woodpile",
     "--gen-mesh-canopy", "--gen-mesh-haystack", "--gen-mesh-dock",
-    "--gen-mesh-pergola",
+    "--gen-mesh-pergola", "--gen-mesh-chimney",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",
     "--gen-mesh-mailbox", "--gen-mesh-tombstone", "--gen-mesh-crate",