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/client/client_send.c

@@ -72,7 +72,7 @@ static void log_server_rejection(const char* context) {
  * continues past an unreadable subdirectory so the readable tree transfers, and
  * always reports the partial transfer (exit 23); this only decides whether the
  * deletion phase is skipped.  Returns true when deletion may proceed. */
-static bool ignore_errors_allows_delete(const Config* config, bool had_io_error) {
+bool ignore_errors_allows_delete(const Config* config, bool had_io_error) {
   return !had_io_error || (config && config->ignore_errors);
 }