feat(editor): add --gen-mesh-hitching-rail multi-post variant

74th procedural mesh primitive. Long horizontal hitching
bar held up by N evenly-spaced vertical posts. Distinct
from --gen-mesh-hitching-post (just 2 posts + bar) — this
is the longer multi-post variant for taverns, stockyards,
racecourse parking, market days, festival hitching lines.

Posts auto-spaced from -L/2 to +L/2, inset by postW so the
end posts align with the rail tips. Bar spans the full
length on top of the post tier.

Watertight under weld (verified 90 manifold edges, 0
boundary, 0 non-manifold). Default 4-post / 4 m rail at
1.2 m mount height fits typical mount silhouettes.

Milestone: kArgRequired now reaches 400 documented flags.

tools/editor/cli_arg_required.cpp

@@ -58,6 +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-camp-pack", "--gen-blacksmith-pack", "--gen-village-pack",
     "--gen-temple-pack", "--gen-graveyard-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",