fix(p7-cli-namespace): address c-review (setfacl -m mangled to --threads; two -s tests lost intent; label/doc sweeps)

- revert over-eager replacement of setfacl -m in test_features.py
- use --chunk-serialization (+ set dirs) in the two append/append-verify
  chunk-serialization rejection unit tests so they exercise the real check
- rename archive-negation integration test (--no-preserve is inert under
  archive because devices/specials force metadata)
- update stale (-c)/(-m)/(-s)/(-f) display labels and RSYNC_COMPAT -c/-m refs
- document the --no-perms negation limitation in the Wave A note

tests/integration/test_stop.py

@@ -234,8 +234,8 @@ class TestStopDelete:
         result, _ = run_client(source, dest, flags=["--threads", "--delete", "--stop-at=now+0s"],
                                port=shared_server.port)
         assert result.returncode == 0, \
-            f"-m --delete immediate stop failed: {(result.stderr or result.stdout)[:400]}"
+            f"--threads --delete immediate stop failed: {(result.stderr or result.stdout)[:400]}"
         received = get_dest_received_dir(dest, source)
         mismatches, missing = verify_transfer(source, received)
         assert not mismatches and not missing, \
-            f"-m --delete wiped source mirrors: missing={missing} mismatches={mismatches}"
+            f"--threads --delete wiped source mirrors: missing={missing} mismatches={mismatches}"