feat(protocol): add optional STATUS_ERROR_DETAIL rejection reason (2.21.0)

Today a server rejection sends a bare STATUS_ERROR and the reason only
reaches the server log, so the client cannot say why a transfer was
refused.  Add an optional, bounded server->client error-detail frame:

  - Status gains STATUS_ERROR_DETAIL appended LAST so existing wire
    values are unchanged.
  - send_error_detail(fd, msg) sends STATUS_ERROR_DETAIL followed by the
    existing length-prefixed string primitive, slicing over-long messages
    to MAX_ERROR_DETAIL_BYTES (4096).
  - receive_status() (and the timed/keepalive status readers) always
    consume the detail body and map the status back to STATUS_ERROR,
    capturing the text into a thread-local buffer exposed by
    protocol_last_error(); a bare STATUS_ERROR leaves it cleared.  Every
    existing call site keeps working and the stream cannot desync.
  - Upgrade the daemon module gate / config validation (config.c), the
    final transfer failure (server.c) and receiver-side path/node
    validation (file_receive.c) to send a concrete reason; surface it on
    the client in client_send.c/config.c.
  - Bump PROTOCOL_VERSION to 2.21.0 (CMake VERSION, CHANGELOG, docs) and
    update the pinned config wire golden hash / CLI-version tests.
  - Add tests/test_protocol_error.c covering mapping+capture, the
    over-long bound, bare-error clearing, and thread-locality.

README.md

@@ -582,7 +582,7 @@ before the module list, before authentication, and the connecting peer address
 
 ## Protocol and Security
 
-FastSync protocol version `2.20.0` is shared by the client and server. The
+FastSync protocol version `2.21.0` is shared by the client and server. The
 current protocol is sender-driven and includes configuration negotiation,
 including the maximum allocation limit, incremental checks, checksums,
 manifests, keep-alives, abort handling, per-file remove-source results, and