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

TODO.md

@@ -25,6 +25,13 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-02: `photo.exif()` returns every EXIF tag in the file as `ExifTags`,
+  keyed by tag name, each as exifreader decodes it, not only the thirteen common
+  fields (issue 156). The embedded thumbnail's tags are under `Thumbnail`,
+  without the thumbnail image. The thirteen typed methods stay, each picking its
+  field from the tags `exif()` returns, typed as in `PhotoExif`. The example
+  script's JSON files now carry every tag.
+
 - 2026-10-01: The content cache no longer looks for a live photo that an earlier
   version cached as one ZIP (issue 151). When the cache opens, a live photo's
   file that no JSON file names is now always left alone.