Learn's DelayedDeletions, unchanged, with its test, so that Train's
workouts can be deleted with an undo as Learn's items are. Learn's
deletions import it from halkyon_app now.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>