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

test/library/content-library.test.ts

@@ -686,6 +686,10 @@ describe("Photo save path, local copy, content and EXIF", () => {
         await lib.close();
     });
 
+    // The build's type check, not this test, makes sure `Photo` has a method
+    // for every `PhotoExif` field, whatever the fixtures hold: `Photo`
+    // implements a type with one method per field. This test checks that each
+    // method gives the same value as exif().
     it.each([
         ["JPEG", JPEG_WITH_EXIF],
         ["HEIC", HEIC_WITH_EXIF],