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

`Photo` gains thirteen async methods, one per `PhotoExif` field and named after it: `make()`, `model()`, `lensModel()`, `dateTimeOriginal()`, `offsetTimeOriginal()`, `exposureTime()`, `fNumber()`, `iso()`, `focalLength()`, `orientation()`, `gpsLatitude()`, `gpsLongitude()` and `gpsAltitude()`. Each calls `exif()` and returns its one field, or `undefined` when the file lacks it. `exif()` is unchanged.

`Photo` implements a type built from `PhotoExif`'s keys, so the type check fails when a field has no method. Each call reads the original again; a caller that wants several fields calls `exif()` once.

Model: opus-5-5

TODO.md

@@ -25,6 +25,16 @@ 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. `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
   download directory, which defaults to `photos` in the working directory (issue