feat(editor): add --gen-garden-pack composite

Sixth themed mesh pack after camp / blacksmith / village /
temple / graveyard. Garden-courtyard scene emitting a
7-primitive monastery / kitchen-garden / small park layout:

  • pergola — vine-arbor frame
  • fountain — centerpiece
  • stone-bench — seating
  • shrine — decorative focal
  • beehive — productive prop
  • scarecrow — rural touch
  • well — water source

Together these form a recognizable contemplative-garden /
small park / monastery courtyard when arranged in a zone.

Composite-pack catalogue now: camp + blacksmith + village
+ temple + graveyard + garden = 6 themed mesh packs.
--list-packs surfaces 11 total composite flags.

tools/editor/cli_arg_required.cpp

@@ -61,6 +61,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-hitching-rail",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
+    "--gen-garden-pack",
     "--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",