feat(editor): add --gen-mesh-rune-stone standing monolith

91st procedural mesh: a wide flat base block with a tall
narrow monolith standing on top. Reads as a small carved
standing-stone marker.

Distinct from existing ritual-prop primitives:
  • --gen-mesh-altar      — table-shaped, has flat top
  • --gen-mesh-shrine     — multi-tier with cap
  • --gen-mesh-tombstone  — narrower, curved-top silhouette
  • --gen-mesh-pillar     — round column, no base block
  • --gen-mesh-statue     — has figure on top

Useful for: druid groves (boundary markers), witch shrines,
ancient ruins (pre-civilization monuments), graveyard
boundary stones, faction territory markers, Stonehenge-style
ring formations (use 4-8 instances around a center point).

48 verts / 24 tris from two simple boxes — minimal vertex
budget, suitable for placing in dense clusters.

tools/editor/cli_arg_required.cpp

@@ -120,6 +120,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-standing-torch", "--gen-mesh-scroll-case",
     "--gen-mesh-stove", "--gen-mesh-well-pail",
     "--gen-mesh-mug", "--gen-mesh-mortar-pestle",
+    "--gen-mesh-rune-stone",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-garden-pack", "--gen-dock-pack", "--gen-tavern-pack",