feat(editor): add --gen-mesh-canopy market-stall awning

56th procedural mesh primitive. Builds a rectangular awning
on 4 corner posts with a flat overhead panel and optional
drape lips hanging down from each edge:

  • 4 corner posts inset by postR so their outer faces line
    up flush with the panel edges above
  • flat panel slab spanning the full footprint, sitting on
    top of the posts
  • optional drape lips on all four panel edges (set drape=0
    for a clean flat-top canopy with no hanging fabric)

All axis-aligned boxes; the no-drape variant is fully
watertight (verified via --info-mesh-stats --weld). Useful
for market-square stalls, vendor stations, alchemist booths,
caravan rest stops, and tournament viewing platforms.

tools/editor/cli_arg_required.cpp

@@ -50,6 +50,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-shrine", "--gen-mesh-totem", "--gen-mesh-cage",
     "--gen-mesh-throne", "--gen-mesh-coffin", "--gen-mesh-bookshelf",
     "--gen-mesh-tent", "--gen-mesh-firepit", "--gen-mesh-woodpile",
+    "--gen-mesh-canopy",
     "--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",