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/extension.h

@@ -0,0 +1,70 @@
+/* extension.h - BEP-10 transport, BEP-9 metadata, and BEP-11 PEX codecs. */
+#ifndef NAUT_EXTENSION_H
+#define NAUT_EXTENSION_H
+
+#include "naut/common.h"
+#include "naut/tracker.h"
+
+#define NAUT_EXT_UT_METADATA 1
+#define NAUT_EXT_UT_PEX      2
+#define NAUT_METADATA_BLOCK  (16u * 1024u)
+#define NAUT_METADATA_MAX    (4u * 1024u * 1024u)
+
+typedef struct {
+    uint8_t  ut_metadata;
+    uint8_t  ut_pex;
+    uint32_t metadata_size;
+    uint32_t reqq;
+    uint16_t port;
+} naut_ext_handshake;
+
+/* Build a complete peer-wire extended message (length, message ID 20,
+ * extension ID, payload). The returned buffer is malloc-owned. */
+naut_err naut_ext_build_handshake(uint8_t ut_metadata_id, uint8_t ut_pex_id,
+                                  uint32_t metadata_size, uint16_t port,
+                                  uint8_t **out, size_t *out_len);
+naut_err naut_ext_parse_handshake(const uint8_t *payload, size_t len,
+                                  naut_ext_handshake *out);
+
+typedef enum {
+    NAUT_METADATA_REQUEST = 0,
+    NAUT_METADATA_DATA = 1,
+    NAUT_METADATA_REJECT = 2
+} naut_metadata_type;
+
+typedef struct {
+    naut_metadata_type type;
+    uint32_t piece;
+    uint32_t total_size;
+    const uint8_t *data;
+    size_t data_len;
+} naut_metadata_msg;
+
+naut_err naut_metadata_build(uint8_t ext_id, naut_metadata_type type,
+                             uint32_t piece, uint32_t total_size,
+                             const void *data, size_t data_len,
+                             uint8_t **out, size_t *out_len);
+naut_err naut_metadata_parse(const uint8_t *payload, size_t len,
+                             naut_metadata_msg *out);
+
+typedef struct {
+    naut_peer_addr *added;
+    uint8_t        *added_flags;
+    size_t          num_added;
+    naut_peer_addr *dropped;
+    size_t          num_dropped;
+} naut_pex_msg;
+
+naut_err naut_pex_build(uint8_t ext_id, const naut_pex_msg *msg,
+                        uint8_t **out, size_t *out_len);
+naut_err naut_pex_parse(const uint8_t *payload, size_t len, naut_pex_msg *out);
+void     naut_pex_free(naut_pex_msg *msg);
+
+/* Fetch and SHA-1 verify a v1 torrent's raw info dictionary from one peer.
+ * The returned buffer is malloc-owned. */
+naut_err naut_metadata_fetch(const naut_peer_addr *peer,
+                             const uint8_t info_hash[20],
+                             const uint8_t peer_id[20],
+                             uint8_t **info, size_t *info_len);
+
+#endif /* NAUT_EXTENSION_H */