M3: Go to main component names a library copy with no link

A file imported from another app can hold library copies in its library
store with no record of the library they came from. On an instance of
one, the command took the local path and failed in selecting it with
"isn't on a page (it is a library component)", which says nothing about
what to do. It now says the copy has no link to its library and that
enabling the library reaches it.

Found on a real imported file, where copies of the same library set were
cached more than once and only one of them could be linked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/assets/library/tests.rs

@@ -458,6 +458,11 @@ fn going_to_a_library_main_opens_the_library_file() {
     let e = s.execute("component.goToMain", &json!({"id": card})).unwrap_err();
     assert!(e.to_string().contains("isn't at"), "{e}");
     assert_eq!(s.docs.len(), 1);
+
+    let NodeKind::Instance { component: copy, .. } = node(&s, card).kind else { panic!("not an instance") };
+    s.doc_mut().unwrap().libraries_mut().components.retain(|r| r.root != copy);
+    let e = s.execute("component.goToMain", &json!({"id": card})).unwrap_err();
+    assert!(e.to_string().contains("no link to its library"), "an imported copy without its record: {e}");
     let _ = std::fs::remove_dir_all(&d);
 }
 

crates/engine/src/cmd/component.rs

@@ -207,6 +207,13 @@ pub fn specs() -> Vec<CommandSpec> {
                     let library = library.to_string();
                     return go_to_library_main(s, &library, &name, key);
                 }
+                if doc.is_remote(component) {
+                    return Err(EngineError::Disabled(
+                        cmd.into(),
+                        "the main component is a library copy with no link to its library (it came with an imported file): enable that library to reach it"
+                            .into(),
+                    ));
+                }
                 super::edit::select_and_page(s, vec![component])?;
                 let ds = s.ds_mut()?;
                 if let Some(b) = ds.doc.world_bounds(component) {