feat(editor): add --gen-mesh-banner pole + flag primitive

Composite banner: 12-segment vertical pole cylinder with top
and bottom caps + a rectangular flag attached at the top of
the pole, draped along -Z. The flag is two-sided (front faces
+X, back faces -X) so it reads from both viewing angles
without needing backface-culling-disabled materials.

Defaults: poleH=3, poleR=0.05, flagW=0.8, flagH=1.2. Useful
for guild halls, faction territory markers, military camps,
parade dressing. Brings the procedural mesh primitive set
to 31.

tools/editor/main.cpp

@@ -196,6 +196,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-statue", "--gen-mesh-altar", "--gen-mesh-portal",
         "--gen-mesh-archway", "--gen-mesh-barrel", "--gen-mesh-chest",
         "--gen-mesh-anvil", "--gen-mesh-mushroom", "--gen-mesh-cart",
+        "--gen-mesh-banner",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",