docs(parity): correct delete-delay budget, fuzzy, and test-review gaps

- --delete-delay: state that the reported count advances on actual removal
  while --max-delete is charged at plan/snapshot time (defer_add/planned).
  A new differential shows rsync instead charges on actual removals and
  recursively removes a queued directory, so the row moves to Caveat
  (matrix 111/13/33) and the residual is pinned by tests.
- Add a deterministic unit test (plan-time budget charge), a FastSync
  integration test (byte-barrier refill + --max-delete), and an rsync
  differential for a refilled deferred directory.
- Soften the --fuzzy summary: the tree is byte-exact by design, so it is
  pinned by the threshold suite, not a byte-level differential.
- README: describe what -m parity actually selects; fix the allowlist
  example to the real max_delete entry.
- xdist-safe delete-timing fixture names (timing and --threads mode).
- Renumber the duplicate HANDOFF item 9 to 10; add the missing final
  newline to test_checksum.c.
- Expose ignore_errors_allows_delete and unit-test the deletion gate
  without a privileged source directory; update the stale deleted-count
  doc comment.

No production behavior changes.

src/shared/delete_plan.h

@@ -77,8 +77,10 @@ int delete_plan_session_receive(DeletePlanSession* session, const Config* config
 DeleteCommitResult delete_plan_session_commit(DeletePlanSession* session, const Config* config);
 /* True once the shared --max-delete budget stopped part of a deletion. */
 bool delete_plan_session_limit_reached(const DeletePlanSession* session);
-/* Number of destination entries the session's plans removed (or, for
-   --delete-delay, snapshotted for removal), for the end-of-transfer stats. */
+/* Number of destination entries the session actually removed, for the
+   end-of-transfer stats.  For --delete-delay this excludes a snapshotted entry
+   that survived (e.g. a refilled directory that failed ENOTEMPTY), even though
+   that entry already consumed --max-delete budget at snapshot time. */
 size_t delete_plan_session_deleted(const DeletePlanSession* session);
 /* Install an observer invoked for every destination-relative path the session
    truly removes (including the deferred --delete-delay commit), so the receiver