quak backup writes each original's EXIF, XMP and dimensions into its JSON (closes #167)

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

test/exif-heic.ts

@@ -1,7 +1,7 @@
 /**
  * `exif.heic`, beside this file: a real 64x64 HEIC whose EXIF holds the same
- * values as the hand-built JPEG in `library/content-library.test.ts`, for the
- * tests of `exif()` and `backup-metadata --exif`.
+ * values as the hand-built JPEG in `exif-jpeg.ts`, for the tests of `exif()`,
+ * `backup-metadata --exif` and the image metadata `quak backup` records.
  *
  * It was made once, in a throwaway node:22-alpine container (Alpine 3.23.3),
  * with libheif 1.23.0 and exiftool 13.55: