feat(editor): add --gen-mesh-throne regal-seat primitive

5-box composite: low pedestal slab + seat block + tall
vertical backrest at -Z + 2 small armrests on the +X/-X
sides. Pedestal is wider than the seat for a stable
foundation, seat thickness is 30% of total seat height.

Defaults: seatW=0.8, seatH=0.5, backH=1.5, pedSize=1.2.
Useful for throne rooms, hero seats, judgement halls,
royal court set dressing. Brings the procedural mesh
primitive set to 37.

tools/editor/main.cpp

@@ -123,6 +123,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-anvil", "--gen-mesh-mushroom", "--gen-mesh-cart",
         "--gen-mesh-banner", "--gen-mesh-grave", "--gen-mesh-bench",
         "--gen-mesh-shrine", "--gen-mesh-totem", "--gen-mesh-cage",
+        "--gen-mesh-throne",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",