feat(editor): add --gen-mesh-mug tankard primitive

87th procedural mesh: drinking mug / tankard built from a
closed cylindrical body plus a thin side handle slab. The
handle is positioned on +X with its inner face flush against
the body and centered vertically.

Without rotation the handle is a solid box rather than a
real C-loop, so silhouette reads as a tankard handle rather
than a delicate ring — appropriate for the chunky-prop
aesthetic favored by the existing primitive set. Distinct
from --gen-mesh-chalice (3-tier goblet, no handle) and
--gen-mesh-well-pail (top horizontal handle bar).

Useful for tavern / banquet hall / kitchen dressing, inn
table props, and the eventual --gen-tavern-pack-2 follow-on
(once enough drinking-vessel primitives exist to justify a
dedicated drinkware composite).

86 verts / 68 tris at default 14 sides — a single batch
with shared body + handle texture coords.

tools/editor/cli_arg_required.cpp

@@ -72,6 +72,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-lantern", "--gen-mesh-chalice",
     "--gen-mesh-standing-torch", "--gen-mesh-scroll-case",
     "--gen-mesh-stove", "--gen-mesh-well-pail",
+    "--gen-mesh-mug",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-garden-pack", "--gen-dock-pack", "--gen-tavern-pack",