simplify: joining in modelcraft-mesh::join, shared label helper, run_script returns step outputs

Weld and face-to-face check move from the mesh.block command to
modelcraft_mesh::join (bounding-box-limited weld, grid-binned hanging-node
test), for node merging and imports to reuse. mesh.block and
mesh.tet.generate share fresh_labels. The block map evaluates each edge
curve once per lattice parameter; vector helpers come from vec3; tet10 and
hex20 mid-side nodes follow ElementKind::edges. Session::run_script returns
each step's output, which the oracle reads instead of its own loop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/ui-egui/src/lib.rs

@@ -253,7 +253,7 @@ impl ModelApp {
 
     /// Run a script (a JSON array of `{command, params}`), stopping at the first error.
     pub fn run_script(&mut self, script: &Value) -> Result<(), String> {
-        let r = self.session.run_script(script).map_err(|e| e.to_string());
+        let r = self.session.run_script(script).map(|_| ()).map_err(|e| e.to_string());
         self.model_changed();
         match &r {
             Ok(()) => self.set_status("Script done", false),