Photo: one async method per EXIF field (closes #148)

Thirteen methods on Photo, make() through gpsAltitude(), each named after
its PhotoExif field and typed by it. Each calls exif() with the same opts
and returns that one field, or undefined when the file lacks it. A test
checks, on the JPEG fixture and on test/exif.heic, that every field
exif() returns has a method giving the same value. README and TODO.md
list them.

Model: opus-5-5

TODO.md

@@ -25,6 +25,14 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-01: A `Photo` has one async method for each field of `exif()`, named
+  and typed as in `PhotoExif`: `make()`, `model()`, `lensModel()`,
+  `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.
+
 - 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