Merge: answer Android's memory warnings, and stop reporting a lost root as an empty library

FR-PLAT-AND-5 in full, FR-PLAT-AND-2 in part -- the recovery is built and
live for Nextcloud roots, the SAF cause it names does not exist yet.

FR-PLAT-AND-4 and FR-PLAT-AND-6 are not here, both blocked behind the
same gap: assemble-apk.sh compiles no Java, so the APK cannot carry a
Service or a FileProvider. The container has JDK 17 and build-tools 36;
the build step is what is missing.

Verified: fmt, clippy --workspace --all-targets -D warnings, and 1043
tests across dr-catalog, dr-sync, dr-sync-folder, dr-sync-nextcloud,
dr-plat and dr-ui. The aarch64 target was checked before the branch was
finished but not after; no device was available.

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

core/dr-catalog/src/lib.rs

@@ -65,7 +65,7 @@ pub use query::{Query, Sort};
 pub use rating::{Judgement, MAX_RATING};
 pub use scan::{DirAction, DirState, EntryAction, ScanOutcome};
 pub use trash::{TrashedImage, TRASH_DIR};
-pub use walk::{ensure_root, scan_root, RootKind, ScanProgress, ScanReport};
+pub use walk::{ensure_root, mark_root_offline, scan_root, RootKind, ScanProgress, ScanReport};
 
 /// One row of the library grid.
 ///