Each file's JSON gains imageMetadata, what extractImageMetadata finds in
the stored original (for a live photo, its image), or the reason the
read failed in imageMetadataError; a failed read fails neither the file
nor the run. A video is not read. An original is read when the run
stores it or when its JSON has neither field; otherwise the field is
carried over from that JSON, so a run does not read every original
again. The hand-built JPEG fixtures move to test/exif-jpeg.ts so the
backup tests can use them.
Judgement call: an original with nothing to record gets imageMetadata
{} instead of no field, so it is not read again on every run.
Model: opus-5-5
