feat(editor): add --gen-mesh-scarecrow farm-prop primitive

54th procedural mesh: 5-box cruciform farm pest deterrent —
vertical body post, horizontal cross arm, head box on top of
the body, brimmed hat box that's wider than the head, and a
narrower-but-taller crown box on top of the hat brim. The
cross silhouette + brimmed-hat read as a scarecrow without
needing rotated geometry.

Useful for crop fields, abandoned villages, harvest set
dressing, ritual circles, fields after a battle. Defaults
to 1.80m body × 1.40m arm span (~2.18m total height).

tools/editor/main.cpp

@@ -162,6 +162,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-mailbox", "--gen-mesh-tombstone", "--gen-mesh-crate",
         "--gen-mesh-stool", "--gen-mesh-cauldron", "--gen-mesh-gate",
         "--gen-mesh-beehive", "--gen-mesh-weathervane",
+        "--gen-mesh-scarecrow",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",