shift_iso writes back only a time it fully reads (a date alone, or HH:MM
with optional seconds and fraction, an optional zone, nothing after);
iso_seconds stays lenient for sorting and rules. photo.setCaptureTime
leaves such photos as they were and counts them as `unreadable`, and a
set that would move other photos by an unreadable time is refused; alone,
the photo gets the typed time. The dialog's preview says the photo's time
can't be read, OK says what was left, any error keeps the dialog open
(button and ui.dialog.confirm), and Set mode doesn't start from an
unreadable time.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
