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