feat(identity): implement --copy-as USER[:GROUP] safe subset (P7 Wave E)

Force the receiver to apply the requested owner/group to every written
entry through the confined fd-relative identity path instead of switching
the process credentials (unsafe for the multithreaded receiver).  An
unprivileged receiver refuses the transfer up front in server_module_gate,
before STATUS_OK, so no data is written with the wrong ownership.

- new Config fields copy_as_set/copy_as_uid/copy_as_gid + defaults
- identity_parse_copy_as (name/@N/* resolution, primary-gid default,
  gid==uid fallback for numeric ids with no passwd entry); implies -M
- identity snapshot + highest-priority forcing in identity_resolve_targets
- identity_copy_as_refused() helper
- trailing config-frame block (presence int + two int32 ids, >=0 checked)
- PROTOCOL_VERSION 2.17.0 -> 2.18.0; version-sensitive tests updated
- unit tests for parse + wire round-trip/negative-id rejection
- integration TestCopyAs: unprivileged refusal + root chown assertion
- RSYNC_COMPAT.md --copy-as row updated (safe subset + divergence); README
  protocol version refreshed

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=2.17.0 (the current PROTOCOL_VERSION) is accepted and the
+        """--protocol=2.18.0 (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=2.17.0"],
+        result, _ = run_client(source, dest, flags=["--protocol=2.18.0"],
                                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 ("2.15.0", "2.16.0", "216", "31"):
+        for bad in ("2.15.0", "2.16.0", "2.17.0", "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"