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.
scan_chunk resolves album folders whose (artist, album) sorts after a
cursor, up to a limit, returning (new_cursor, imported, skipped, done).
scan_library becomes the unbounded case (limit=None), so its behavior and
tests are unchanged. Idempotent: all writes are ON CONFLICT, so an
overlapping/repeated cursor is safe. Enables chunked scanning in the loop.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jonathan