feat(editor): add --gen-mesh-urn 4-tier pottery vessel

79th procedural mesh primitive. Vertical urn built from 4
stacked tiers using the new addClosedCylinderY helper:

  • foot — wide short cylinder (the ground-meeting base)
  • body — tall main cylinder (the storage volume)
  • neck — narrow constriction
  • lip — slightly wider rim cap

Each tier is an independent watertight Y-axis cylinder with
its own ±Y end caps; they're stacked vertically with each
sitting on the previous tier's top.

Useful for temple offerings, mausoleum interiors, kitchen
storage, alchemist labs, funerary scenes, witch-hut detail.
First handler to consume the new addClosedCylinderY helper —
demonstrates the pattern for future cylindrical primitives
(candle, lantern, scroll case, well-pail, etc.).

Watertight under weld (verified 432 manifold edges, 0
boundary, 0 non-manifold).

tools/editor/cli_arg_required.cpp

@@ -60,7 +60,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-stone-bench", "--gen-mesh-mine-cart",
     "--gen-mesh-hitching-rail", "--gen-mesh-pillar-row",
     "--gen-mesh-statue-base", "--gen-mesh-bird-bath",
-    "--gen-mesh-planter-box",
+    "--gen-mesh-planter-box", "--gen-mesh-urn",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-garden-pack", "--gen-dock-pack", "--gen-tavern-pack",