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.

tests/integration/README.md

@@ -38,11 +38,17 @@ FastSync mirrors the absolute source path under its receive root (see
 # Fast subset that guards the ✅ surface on pull requests
 python3 -m pytest tests/integration/test_differential_parity.py -n 4 --dist=load -m parity_ci
 
-# Full set (all ✅ cases plus the documented ⚠️/❌ residuals)
+# Full differential case table (`_CASES`): every row is marked `parity`, and a
+# case with an allowlisted residual in `parity_caveats.py` is included too.
 python3 -m pytest tests/integration/test_differential_parity.py -n 4 --dist=load -m parity
 ```
 
-The suite skips cleanly when `rsync` is not installed.
+`-m parity` selects only the `_CASES` table in this module. Differential
+coverage for options outside that table (`--temp-dir`, `--delay-updates`,
+`--dry-run`, `--fuzzy`, the basis-dir options, `-M` over daemon/TCP, and
+receiver filter-protect) lives in dedicated modules (`test_option_parity.py`,
+`test_parity_blockers.py`, `test_parity_quickwins.py`, ...) and is not part of
+this gate. The suite skips cleanly when `rsync` is not installed.
 
 ## Allowlist (`parity_caveats.py`)
 
@@ -52,10 +58,11 @@ Each entry maps a case id to the aspects that may differ (`tree`, `stdout`,
 
 ```python
 CAVEATS = {
-    "min_size": {
-        "tree": "recursive transfer does not create a source directory that "
-                "becomes empty after --min-size filtering. ref: RSYNC_COMPAT.md "
-                "`-d/--dirs` row and completion-wave residual.",
+    "max_delete": {
+        "tree": "which destination extras survive a partial --max-delete abort "
+                "is deletion-order dependent and unspecified; rc=25 and the "
+                "number of survivors match rsync. ref: RSYNC_COMPAT.md "
+                "`--max-delete=NUM` row.",
     },
 }
 ```