The segment scan behind `backup-metadata --exif` now checks every
segment length against the bytes that remain and stops on lengths
under 2, so a truncated or corrupt original can neither throw nor loop.
A malformed or unparseable EXIF segment is recorded as
`imageMetadata.exifError`, and a failure to read the original as
`imageMetadataError` in the per-file JSON, instead of the field being
silently left out. Tests use short hand-built byte arrays.
Model: opus-5-5
