Introduce libfreemkv::scan_iso(path, opts) -> (Disc, Box<dyn SectorSource>),
the file-backed counterpart to DiscSession::scan. It is the single place
that opens a FileSectorSource, reads its capacity, and runs Disc::scan_image,
returning the scanned Disc plus a reusable reader over the same image so
consumers stop hand-rolling that triple.
Add an integration test that materialises a minimal synthetic UDF image to a
real file, asserts scan_iso matches the manual open+scan_image composition,
and confirms the returned reader is still usable (capacity + sector read).
Also covers open-failure and scan-failure error propagation.
Matthew Jackson
6f53767e8b