exif() returns every EXIF tag in the file (closes #156)

`photo.exif()` now returns every EXIF tag in the file, as the owner ruled, typed `ExifTags`: each tag keyed by name with `exifreader`'s `id`, `value`, `description` and `computed`. A tag with no name is keyed `undefined-` plus its number, and the embedded thumbnail's tags sit under `Thumbnail`. The thirteen typed methods stay, with the same names and types; each now picks its field from `exif()`'s tags. GPS latitude and longitude are worked out from the GPS tags and their reference tags, and a position with no reference tags gives neither. `backup-metadata --exif` output is unchanged.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -101,7 +101,8 @@ requires one, and writes:
   photo its image, its video and the `.livephoto.json` file naming them
 - beside each original, a JSON file named after it with `.json` added, for
   example `2026-03-01.12345.jpg.json`: the photo's record (`photo.record()`)
-  without its cache paths, and its EXIF fields (`photo.exif()`) under `exif`
+  without its cache paths, and every EXIF tag of the photo (`photo.exif()`)
+  under `exif`
 - `albums/<collectionID>.json` for each album: its `collectionID`, its `name`,
   and under `savePaths` the save paths of its photos relative to `dir`, newest
   first
@@ -773,21 +774,25 @@ These async methods may download:
 - `await photo.thumbnail(opts?)` → `{ path, bytes }`.
 - `await photo.content(opts?)` → `Uint8Array` — the original's bytes, read
   through `original()`; for a live photo, its image's.
-- `await photo.exif(opts?)` → `PhotoExif` — `make`, `model`, `lensModel`,
-  `dateTimeOriginal`, `offsetTimeOriginal`, `exposureTime`, `fNumber`, `iso`,
-  `focalLength`, `orientation`, `gpsLatitude`, `gpsLongitude` and `gpsAltitude`,
-  each absent when the file lacks it. GPS values are signed decimal degrees and
-  metres. `dateTimeOriginal` is the camera's clock reading held in the `Date`'s
-  UTC fields; `offsetTimeOriginal`, when present, is that clock's offset from
-  UTC. EXIF is read from any image format exifreader reads (such as JPEG,
-  HEIC/HEIF, AVIF, PNG, WebP and TIFF), a live photo's image included. Any other
-  original gives `{}`, and a video gives `{}` without being downloaded.
+- `await photo.exif(opts?)` → `ExifTags` — every EXIF tag in the file, keyed by
+  tag name, each as exifreader decodes it, with its `id`, `value`, `description`
+  and `computed` value: for example `Make` is
+  `{ id: 271, value: ["Canon"], description: "Canon", computed: "Canon" }`. A
+  tag exifreader has no name for is keyed `undefined-<tag number>`. The embedded
+  thumbnail's tags are under `Thumbnail`, so they cannot hide the main image's
+  tags of the same name; the thumbnail image itself is left out. EXIF is read
+  from any image format exifreader reads (such as JPEG, HEIC/HEIF, AVIF, PNG,
+  WebP and TIFF), a live photo's image included. Any other original gives `{}`,
+  and a video gives `{}` without being downloaded.
 - `await photo.make(opts?)`, and likewise `model()`, `lensModel()`,
   `dateTimeOriginal()`, `offsetTimeOriginal()`, `exposureTime()`, `fNumber()`,
   `iso()`, `focalLength()`, `orientation()`, `gpsLatitude()`, `gpsLongitude()`
-  and `gpsAltitude()` → one field of `exif()` each, typed as in `PhotoExif`, or
-  `undefined` when the file lacks it. Each calls `exif()` with its `opts`, so
-  each call reads the original again.
+  and `gpsAltitude()` → one common field each, picked from the tags `exif()`
+  returns and typed as in `PhotoExif`, or `undefined` when the file lacks it.
+  GPS values are signed decimal degrees and metres. `dateTimeOriginal()` is the
+  camera's clock reading held in the `Date`'s UTC fields;
+  `offsetTimeOriginal()`, when present, is that clock's offset from UTC. Each
+  calls `exif()` with its `opts`, so each call reads the original again.
 
 They serve from the on-disk content cache when the bytes are present and
 otherwise fetch through the pools; `original()`, `content()` and `exif()` also
@@ -904,7 +909,7 @@ from a very old client, is stored unchecked.
 - `src/library/records.ts`: `PhotoRecord`, `AlbumRecord`, `LibrarySnapshot`,
   `LibraryChange`
 - `src/library/mlsearch.ts`: `MLDataAPI`, `SimilarResult`
-- `src/exif.ts`: `PhotoExif`
+- `src/exif.ts`: `ExifTags`, `PhotoExif`
 - `src/library/pools.ts`: `RequestPools`, `RequestPoolsOptions`, `BoundedPool`
 - `src/backup.ts`: `BackupOptions`, `BackupResult`, `BackupError`
 - `src/client.ts`: `Client`, `LoginOptions`, `ClientSnapshot`