feat(dry-run): server-contacting --dry-run (protocol 2.21.0)

--dry-run now handshakes with a remote/daemon receiver and reports what
WOULD transfer/skip based on receiver state, mutating nothing on either
side.

- Serialize Config.dry_run into the wire config frame and append
  STATUS_DRY_RUN_TRANSFER to the status enum (no renumbering); bump
  PROTOCOL_VERSION/CMake VERSION/CHANGELOG/golden wire to 2.21.0.
- Receiver: receive_incremental_check_ex runs the normal read-only
  decision and answers STATUS_OK (skip) or STATUS_DRY_RUN_TRANSFER
  (would transfer) with no basis materialization/append/delta/full
  transfer.  All mutation sites are guarded by !dry_run: file store,
  manifest deletes, --mkpath root creation, --delay-updates staging,
  publication, directory-time application, and outcome acks.
- Client: send_dry_run_remote connects, sends the config, checks each
  regular file and prints the would-transfer set + trailer; no file data
  or delete manifest is sent.  Plain local destinations keep the
  client-side manifest.

tests/integration/test_preflight.py

@@ -94,14 +94,14 @@ def _seed_protocol_source(source):
 class TestProtocol:
     @pytest.mark.ci
     def test_protocol_current_version_accepted(self, shared_server):
-        """--protocol=2.20.0 (the current PROTOCOL_VERSION) is accepted and the
+        """--protocol=2.21.0 (the current PROTOCOL_VERSION) is accepted and the
         transfer completes normally."""
         source = os.path.join(TEST_DATA_DIR, "proto_ok_src")
         dest = os.path.join(TEST_DATA_DIR, "proto_ok_dst")
         shutil.rmtree(dest, ignore_errors=True)
         os.makedirs(dest)
         _seed_protocol_source(source)
-        result, _ = run_client(source, dest, flags=["--protocol=2.20.0"],
+        result, _ = run_client(source, dest, flags=["--protocol=2.21.0"],
                                port=shared_server.port)
         assert result.returncode == 0, \
             f"--protocol current run failed: {(result.stderr or result.stdout)[:400]}"
@@ -118,7 +118,7 @@ class TestProtocol:
         shutil.rmtree(dest, ignore_errors=True)
         os.makedirs(dest)
         _seed_protocol_source(source)
-        for bad in ("2.19.0", "2.18.0", "2.17.0", "2.15.0", "2.16.0", "216", "31"):
+        for bad in ("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"