exif(): read HEIF/HEIC originals with exifreader (closes #145)

`photo.exif()` and `quak backup-metadata --exif` now read EXIF through `exifreader`. HEIC/HEIF originals get EXIF, including a live photo's image, as do the other formats `exifreader` reads. It replaces `exif-reader` and the hand-written JPEG scan. `PhotoExif` is unchanged.

The `backup-metadata` dump now holds `exifreader`'s tag output, with unnamed tags keyed `undefined-` plus their number. GPS altitude without a reference counts as above sea level. A latitude or longitude without its hemisphere tag, an unreadable text tag, and a date the parser rejects each give no field.

Licence: `exifreader` is MPL-2.0, used unmodified.

Model: opus-5-5

TODO.md

@@ -25,6 +25,14 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-01: `photo.exif()` and `backup-metadata --exif` read EXIF from
+  HEIC/HEIF originals, a live photo's HEIC image included, as well as JPEG and
+  the other image formats `exifreader` reads (issue 145). `exifreader` replaces
+  `exif-reader` and the JPEG segment scan; `PhotoExif` is unchanged. The `exif`
+  field of `backup-metadata --exif` is now exifreader's tag output, and
+  `exifRaw` holds the whole EXIF block it could not read. The tests use a real
+  HEIC, `test/exif.heic`.
+
 - 2026-10-01: A `Photo` has `savePath`, `isLocal`, `content()`, `exif()`,
   `modifiedAt`, `hash` and `year` (issue 141). `savePath` is where
   `lib.backup()` writes the original under the library's download directory; for