Replace scan_chunk's O(N^2) tree re-walk-and-skip-to-cursor with a
persisted ScanWorkItem worklist: build_worklist walks the tree once at
scan start and inserts one row per album (idempotent via ON CONFLICT),
scan_chunk pops up to `limit` not-done rows by indexed cursor and marks
them done, and clear_worklist drops the scan's rows once drained.
scan_library is reimplemented on top of the worklist (one-shot scan_id,
drain, clear) with an unchanged signature/behavior. main.py's
maybe_run_scan now drives a generated scan.id through Config
(scan.id/scan.inProgress/scan.progress/scan.result) instead of a
scan.cursor string, so a mid-scan library change no longer risks
skipping or re-walking albums.
Jonathan
0115b82867