feat(editor): add --gen-mesh-pillar-row colonnade primitive

75th procedural mesh primitive. Row of N stone pillars
evenly spaced along the X axis. Each pillar is a single
tall rectangular box optionally crowned by a slightly-wider
square cap (capExtra wider per side).

End pillars inset by pillarW so they sit at exactly the
±span/2 ends. Set capH=0 for bare un-crowned columns.

Useful for ruined temples, palace colonnades, dungeon
hallways, dwarven mead-hall arcades, ceremonial entry
walks. Distinct from --gen-mesh-pillar (single column) —
this is the regular-multi-column composite, the third
"scene" primitive after --gen-mesh-crate-stack and
--gen-mesh-gravel-pile.

Watertight under weld (verified 144 manifold edges, 0
boundary, 0 non-manifold). Default 4-pillar / 4 m / 2.5 m
spacing reads as a small temple front porch.

tools/editor/cli_arg_required.cpp

@@ -58,7 +58,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-outhouse", "--gen-mesh-forge",
     "--gen-mesh-archery-target", "--gen-mesh-gravel-pile",
     "--gen-mesh-stone-bench", "--gen-mesh-mine-cart",
-    "--gen-mesh-hitching-rail",
+    "--gen-mesh-hitching-rail", "--gen-mesh-pillar-row",
     "--gen-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-garden-pack",