STEP: bound assembly traversal, name imports after a lone body

A visit budget stops shared sub-assemblies from growing exponentially (with
a test); the loose-solid pass gets its own budget. An Import feature is
named after the root product, else its only body, else the file. Restores
a test function closed off by the rebase.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/tests.rs

@@ -380,6 +380,7 @@ fn timeline_edits_reresolve_references() {
     run(&mut s, "UndoCommand", json!({}));
     run(&mut s, "UndoCommand", json!({}));
     assert!(rel(volume(&mut s), 60.0 * 30.0 * 25.0 - f4 - PI * 9.0 * 25.0) < 1e-3);
+}
 
 /// A 40 × 30 × 20 box exported to a STEP file in a fresh temp directory.
 fn step_box_file(tag: &str) -> std::path::PathBuf {