Refuse a scan whose root has gone, instead of reporting it empty

FR-PLAT-AND-2, and a silent failure on both platforms. `dr_sync::scan`
stepped over a NotFound or PermissionDenied the way it does for a child
that vanished mid-walk -- correct for a child, wrong for the root, where
it ended the walk, returned Ok with nothing in it, and reported a
successful scan of a library that was no longer there.

A lost root is now its own error. The images under it are marked
Availability::Offline per FR-CAT-9 and no catalog row is deleted;
`library::persist` clears the mark per file as each one is listed again,
so a root that comes back needs no repair step.

Partly satisfied rather than closed, and the gap is worth stating.
The recovery half is real and reachable on Android today, because
`map_status` turns Nextcloud's 403 and 404 into it and Nextcloud is how
a phone actually gets a library in this build. The causes the
requirement names -- revocation, reinstall, a removed card -- are
properties of a persisted tree permission, and there is none: SAF does
not exist here, `SourceRef::Document` is constructed only in test
modules, and `LocalStorage` rejects the variant outright. When SAF
lands it becomes a third producer of this error and nothing above it
changes, which is why the discovery belongs in the connector.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-sync-folder/src/tests.rs

@@ -48,13 +48,22 @@ fn names(entries: &[RemoteEntry]) -> Vec<String> {
 
 // --- opening --------------------------------------------------------------
 
+/// TRACES: FR-PLAT-AND-2
 #[test]
-fn a_missing_folder_is_a_configuration_error_not_a_network_one() {
-    // It must not put the app into offline mode: nothing was unreachable, the
-    // account names somewhere that is not a folder.
+fn a_missing_folder_is_an_unavailable_root_not_a_network_failure() {
+    // Still not offline mode — nothing was unreachable over a wire, and
+    // reporting it as a network failure would tell the user to wait for a
+    // connection that is working.
+    //
+    // `RootUnavailable` rather than `Configuration`, because the caller that
+    // has to act on this is the one whose library worked yesterday: an
+    // ejected card is indistinguishable from a mistyped path here, and only
+    // the first of those has a catalog full of ratings to protect.
     let err = FolderBackend::new("/definitely/not/here").unwrap_err();
-    assert!(matches!(err, RemoteError::Configuration(_)), "{err:?}");
+    assert!(matches!(err, RemoteError::RootUnavailable(_)), "{err:?}");
+    assert!(err.indicates_lost_root());
     assert!(!err.indicates_offline());
+    assert!(err.to_string().contains("/definitely/not/here"), "{err}");
 }
 
 // --- listing --------------------------------------------------------------