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/testkit/src/corpus.rs

@@ -167,7 +167,7 @@ pub fn corpus_dir() -> PathBuf {
     workspace_root().join("tests").join("corpus")
 }
 
-fn read(path: &Path) -> Result<String> {
+pub(crate) fn read(path: &Path) -> Result<String> {
     let io = |e: std::io::Error| Error::Io(format!("{}: {e}", path.display()));
     let len = std::fs::metadata(path).map_err(io)?.len();
     if len > MAX_FILE_BYTES {