test(delete): differential + timing regression tests for per-directory delete plans

- Compare --delete-during/--delete-delay final state against rsync 3.4.1.
- Force a mid-transfer failure through a byte-slicing proxy: --delete-during has
  removed the processed directory's extra, --delete-delay has not.
- Create a destination entry while the transfer is in flight: it survives
  --delete-delay's snapshot but is removed by --delete-after's fresh end scan.
- Cover the --delete-delay type-conflict case now matching rsync.

src/server/server.c

@@ -966,8 +966,7 @@ void handler(int file_descriptor) {
          arrived; with the disk writer drained, commit the deferred removals.
          --delete-during already applied its plans on the receive thread. */
       if (context->deferred_plans) {
-        DeleteCommitResult deletion =
-            delete_plan_session_commit(context->deferred_plans, config);
+        DeleteCommitResult deletion = delete_plan_session_commit(context->deferred_plans, config);
         if (deletion == DELETE_COMMIT_ERROR) {
           transfer_ok = false;
         } else if (deletion == DELETE_COMMIT_LIMIT_REACHED) {