feat(editor): add --gen-mesh-watchpost sentry tower primitive

64th procedural mesh primitive. A simple scout/sentry
watchpost from axis-aligned boxes:

  • tall central pole rising from ground to postH
  • square platform slab on top of the pole, wider than
    the pole so it overhangs as the lookout deck
  • 4 corner railing posts on the platform — set
    railingH=0 for a bare deck

Distinct from --gen-mesh-tower (round castle tower with
crenellated battlements). A watchpost is the rough scout/
forward-outpost variant — just wood and rope, no masonry.

Pairs naturally with --gen-mesh-tent / --gen-mesh-firepit
for camp + lookout scenes, and --gen-mesh-fence for
perimeter setups. Watertight under weld (verified
108 manifold edges, 0 boundary, 0 non-manifold).

tools/editor/cli_arg_required.cpp

@@ -53,6 +53,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-canopy", "--gen-mesh-haystack", "--gen-mesh-dock",
     "--gen-mesh-pergola", "--gen-mesh-chimney", "--gen-mesh-bedroll",
     "--gen-mesh-workbench", "--gen-mesh-crate-stack",
+    "--gen-mesh-watchpost",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",
     "--gen-mesh-mailbox", "--gen-mesh-tombstone", "--gen-mesh-crate",