feat(editor): add --gen-mesh-water-trough basin primitive

65th procedural mesh primitive. Open-top rectangular basin
built from 5 axis-aligned boxes (uses every shared helper
from cli_box_emitter):

  • bottom slab spanning full footprint at wallT thickness
  • +X / -X side walls spanning full length
  • +Z / -Z front/back walls spanning the inner length so
    they tuck inside the side walls without overlap

Inner cavity (length-2*wallT × height-wallT × width-2*wallT)
is the open water/feed volume. Useful for stables, farmsteads,
taverns, stockyards, alchemist mixing basins.

Default 1.4 × 0.5 × 0.5 m is a reasonable horse-trough size;
configurable for smaller hand-basins or larger livestock
troughs.

tools/editor/cli_arg_required.cpp

@@ -53,7 +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-watchpost", "--gen-mesh-water-trough",
     "--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",