feat(editor): add --gen-mesh-dock pier-on-stilts primitive

58th procedural mesh primitive. Builds a wooden waterside
dock from axis-aligned boxes:

  • flat plank deck slab spanning length × width at deckT
    thickness, sitting at deckHeight off the ground
  • N pairs of square pilings under the long edges, evenly
    spaced from end to end
  • pilings inset by pilingW so the deck overhangs slightly
    (the standard "boards rest ON TOP of the posts" look)

Distinct from --gen-mesh-bridge which arches OVER an
obstacle — a dock walks straight out from a shoreline on
stilts to the water. Useful for fishing villages, port
warehouses, lake settlements, ferry stations, smuggler
hideouts.

Watertight under weld (--audit-watertight passes): pilings
sit below the deck overhang and don't share corners with it.

tools/editor/cli_arg_required.cpp

@@ -50,7 +50,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-shrine", "--gen-mesh-totem", "--gen-mesh-cage",
     "--gen-mesh-throne", "--gen-mesh-coffin", "--gen-mesh-bookshelf",
     "--gen-mesh-tent", "--gen-mesh-firepit", "--gen-mesh-woodpile",
-    "--gen-mesh-canopy", "--gen-mesh-haystack",
+    "--gen-mesh-canopy", "--gen-mesh-haystack", "--gen-mesh-dock",
     "--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",