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.
