feat(editor): add --gen-mesh-haystack terraced farm haystack

57th procedural mesh primitive. Builds a layered haystack:
N stacked frustums, each smaller than the one below, with
the topmost layer tapering to an apex point. The visible
shelves where each layer overhangs the smaller one above
read as bound straw shocks rather than the smooth
silhouette --gen-mesh-pyramid produces.

Per layer: vertical side wall ring (no taper within a
single layer) plus a top "shelf" annulus where it meets
the next layer up. Final layer is a cone fan to a single
apex vertex. Closed bottom disc keeps the model a solid
for collision baking — verified watertight under weld.

Useful for farms, granaries, stables, harvest-festival
clearings, druid groves with bound bundles. Default 3
layers at 12 sides reads cleanly at typical NPC scale.

tools/editor/cli_arg_required.cpp

@@ -50,7 +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-canopy", "--gen-mesh-haystack",
     "--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",