feat(editor): add --gen-mesh-anvil blacksmith primitive

Composite anvil silhouette: 4-step pedestal (wide base → narrow
waist → wide cap → flat work face), each step a single addBox
call, plus a 4-vertex tapered prism for the horn extending in
+X past the face. The prism's 4 side triangles converge to a
single tip vertex (the horn point), with a sealed base square
behind it.

Defaults: length=1.0, width=0.4, hornLen=0.5, bodyH=0.5. Useful
for blacksmith shops, dwarven forges, weapon-crafting set
dressing. Brings the procedural mesh primitive set to 28.

Added directly to cli_gen_mesh.cpp — proves the modular file
accepts new primitives the same way as extracted ones.

tools/editor/main.cpp

@@ -557,6 +557,7 @@ int main(int argc, char* argv[]) {
         "--gen-mesh-tower", "--gen-mesh-house", "--gen-mesh-fountain",
         "--gen-mesh-statue", "--gen-mesh-altar", "--gen-mesh-portal",
         "--gen-mesh-archway", "--gen-mesh-barrel", "--gen-mesh-chest",
+        "--gen-mesh-anvil",
         "--gen-texture-gradient",
         "--gen-mesh-from-heightmap", "--export-mesh-heightmap",
         "--displace-mesh",