test(parity): assert max_delete invariants, empty the parity allowlist

The max_delete case allowlisted the tree aspect because the surviving extras after a partial --max-delete abort are deletion-order dependent. Under FASTSYNC_PARITY_STRICT a run where the orders coincide was reported as a stale entry (CI failure), while removing the entry made the order-dependent tree mismatch fail. Add a per-case 'compare_tree' flag: max_delete now asserts rc=25 plus the survivor count via extra_check instead of exact tree identity, so the allowlist can be empty. Burn-down reached zero.

tests/integration/README.md

@@ -58,12 +58,8 @@ Each entry maps a case id to the aspects that may differ (`tree`, `stdout`,
 
 ```python
 CAVEATS = {
-    "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.",
-    },
+    # no known residuals at present -- the burn-down reached zero
+    # "some_case_id": {"tree": "documented residual ... ref: RSYNC_COMPAT.md ..."},
 }
 ```