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

`photo.exif()` returns `ExifTags`: every EXIF tag exifreader reads from the file, keyed by tag name, each with exifreader's `id`, `value`, `description` and `computed`. The embedded thumbnail's tags are under `Thumbnail`, without the thumbnail image. A file with no EXIF, or a video, gives `{}`.

The thirteen typed methods stay. Each picks its field from the tags `exif()` returns through `readPhotoExif`, which now takes the tags instead of the bytes. GPS latitude and longitude are worked out from their tags and reference tags, since exifreader's computed position is not among the tags.

Also closes #148.

Model: opus-5-5

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`