feat(basis): rsync quick-check default + FastSync-only --verify-basis

- default basis match is rsync's metadata quick-check (size + mtime; size-only
  drops mtime; -I disables), no mandatory content digest
- new long-only --verify-basis (wire bool, protocol stays 2.28.0) restores the
  strict whole-file content equality
- --copy-dest re-applies source attributes; basis-hit 256 MiB cap removed by
  streaming the copy/hash; basis miss keeps the normal payload bound
- compare/copy/link-dest rows -> caveat; tally 116/13/28

tests/integration/test_preflight.py

@@ -118,8 +118,8 @@ class TestProtocol:
         shutil.rmtree(dest, ignore_errors=True)
         os.makedirs(dest)
         _seed_protocol_source(source)
-        for bad in ("2.22.0", "2.21.0", "2.20.0", "2.19.0", "2.18.0", "2.17.0", "2.15.0", "2.16.0",
-                    "216", "31"):
+        for bad in ("2.27.0", "2.26.0", "2.25.0", "2.24.0", "2.23.0", "2.22.0", "2.21.0", "2.20.0",
+                    "2.19.0", "2.18.0", "2.17.0", "2.15.0", "2.16.0", "216", "31"):
             result, _ = run_client(source, dest, flags=[f"--protocol={bad}"],
                                    port=shared_server.port)
             assert result.returncode != 0, f"--protocol={bad} should be rejected"