feat(editor): add --gen-mesh-planter-box garden flowerbox

78th procedural mesh primitive. Long open-top wooden planter
with a visible soil-fill block:

  • 5-piece basin construction (bottom slab + 4 perimeter
    walls), same arrangement as --gen-mesh-water-trough
  • soil block filling the inner cavity from floor up to
    soilTopFrac × height (default 85%), slightly inset from
    walls so it visually sits "inside" the wood

Distinct from --gen-mesh-water-trough (square basin without
a soil-fill block) — planter-box is the long elongated
garden variant. Useful for window sills, kitchen herb
gardens, balcony planters, monastery cloister-walk
container plants, market-stall produce display trays.

The 6 non-manifold edges (--info-mesh-stats) are at the
4-wall corners — same expected pattern as water-trough,
where the inner walls share corner positions with each
other.

Milestone: kArgRequired now reaches 411 documented flags.

tools/editor/cli_arg_required.cpp

@@ -60,6 +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-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-garden-pack", "--gen-dock-pack", "--gen-tavern-pack",