Name untitled designs after their file (#483)

* fix: name an untitled design after its file on save and open

Design tabs all read "Untitled" because a design's name only ever came
from file.new. Saving a design still under the default name now names it
after the file (bracket.solidcraft -> "bracket"), and opening a file whose
stored name is the default shows its file name. Names chosen by the user
are kept. The rename is part of the save, so it is not an undo step and
does not leave the design marked as changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HWLPLy7ug7URSK5WybjRNa

* test: expect the file's name after saving an untitled design

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HWLPLy7ug7URSK5WybjRNa

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/sketch_attachment_ui_tests.rs

@@ -115,10 +115,16 @@ fn lost_attachment_ghost_rebind_preview_cancel_apply_and_undo_preserve_sketch()
     let repaired = h.app.session.doc.to_json();
     let path = h.dir.join("rebound-face.solidcraft");
     h.app.run("file.save", json!({"path":path})).unwrap();
+    // Saving the untitled design names it after its file; everything else is exact.
+    let named = |doc: &str| {
+        let mut doc = solidcraft_engine::doc::Document::from_json(doc).unwrap();
+        doc.name = "rebound-face".into();
+        doc.to_json()
+    };
     h.app.run("edit.undo", json!({})).unwrap();
-    assert_eq!(h.app.session.doc.to_json(), before);
+    assert_eq!(h.app.session.doc.to_json(), named(&before));
     h.app.run("doc.open", json!({"path":path})).unwrap();
-    assert_eq!(h.app.session.doc.to_json(), repaired);
+    assert_eq!(h.app.session.doc.to_json(), named(&repaired));
     assert_eq!(raw_sketch(&h, sketch), raw);
     assert!(h.app.session.model.result(sketch).unwrap().error.is_none());
 }