Initial commit: Naut-Torrent — from-scratch 10 GbE BitTorrent client

A maintainable, extensible BitTorrent client (C11, Linux/io_uring) targeting
10 GbE saturation. All torrent functionality is built from scratch; liburing
is the only linked third-party dependency on the data path.

Implements Phases 1-7 of the roadmap:
- core: page-aligned buffer pool, MPMC/Treiber queues, bitfields, worker pool
- crypto: SHA-1/256 (SHA-NI + scalar), Merkle (BEP-52), RC4 (MSE)
- bencode/metainfo: zero-copy parser, v1/v2/hybrid .torrent + magnet
- peer: sans-IO wire codec, MSE/PE handshake state machine, BEP-10, ut_metadata, PEX
- piece/storage: block-level multi-peer engine, rarest-first + endgame,
  per-file completion events + single-file relocate (move-as-you-finish)
- tracker/dht: HTTP + UDP (BEP-15) trackers, BEP-5 KRPC iterative lookup
- platform: io_uring reactor (SQPOLL, registered buffers, SEND_ZC)
- surface: versioned RPC, native plugin ABI, sandboxed Lua scripting, nautd/nautctl

Verified against libtorrent (single/multi/hybrid, MSE, magnet-via-DHT, swarm);
unit + interop tests green; ASan/UBSan/TSan clean. Scripting reference in
docs/scripting.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

include/naut/rpc.h

@@ -0,0 +1,45 @@
+/* rpc.h - versioned length-prefixed control protocol and command registry. */
+#ifndef NAUT_RPC_H
+#define NAUT_RPC_H
+
+#include "naut/common.h"
+#include "naut/event.h"
+
+#include <jansson.h>
+
+#define NAUT_RPC_VERSION 1
+#define NAUT_RPC_MAX_PAYLOAD (1u << 20)
+
+typedef enum {
+    NAUT_RPC_REQUEST = 1,
+    NAUT_RPC_RESPONSE = 2,
+    NAUT_RPC_EVENT = 3,
+} naut_rpc_frame_type;
+
+typedef struct naut_rpc_registry naut_rpc_registry;
+typedef json_t *(*naut_rpc_handler)(void *context, const json_t *params,
+                                    naut_err *error);
+
+naut_rpc_registry *naut_rpc_registry_create(void);
+void naut_rpc_registry_destroy(naut_rpc_registry *registry);
+naut_err naut_rpc_register(naut_rpc_registry *registry, const char *method,
+                           naut_rpc_handler handler, void *context);
+void naut_rpc_unregister(naut_rpc_registry *registry, const char *method);
+json_t *naut_rpc_dispatch(naut_rpc_registry *registry, const char *method,
+                          const json_t *params, naut_err *error);
+
+naut_err naut_rpc_send_json(int fd, naut_rpc_frame_type type,
+                            const json_t *payload);
+naut_err naut_rpc_recv_json(int fd, naut_rpc_frame_type *type,
+                            json_t **payload);
+
+int naut_rpc_connect_unix(const char *socket_path);
+
+/* Connect to a UNIX socket and perform one request/response exchange. */
+naut_err naut_rpc_call(const char *socket_path, const char *method,
+                       const json_t *params, json_t **response);
+
+/* Convert an event into the stable JSON event representation. */
+json_t *naut_rpc_event_json(const naut_event *event);
+
+#endif /* NAUT_RPC_H */