Photo implements one method per PhotoExif field

Photo now implements a mapped type with one method per PhotoExif field,
each taking exif()'s options and giving that field's type, so the
build's type check fails when PhotoExif has a field Photo has no method
for, whatever the test fixtures hold. The test comment and TODO.md say
so.

Model: opus-5-5

TODO.md

@@ -30,8 +30,10 @@ declares one.
   `dateTimeOriginal()`, `offsetTimeOriginal()`, `exposureTime()`, `fNumber()`,
   `iso()`, `focalLength()`, `orientation()`, `gpsLatitude()`, `gpsLongitude()`
   and `gpsAltitude()` (issue 148). Each calls `exif()` and returns its one
-  field, or undefined when the file lacks it. A test checks, on the JPEG and the
-  HEIC, that every field `exif()` returns has a method giving the same value.
+  field, or undefined when the file lacks it. `Photo` implements a type with one
+  method per `PhotoExif` field, so the build's type check fails when a field has
+  no method. A test checks, on the JPEG and the HEIC, that each method gives the
+  same value as `exif()`.
 
 - 2026-10-01: Each original's save path is
   `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>` under the library's