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/tracker.h

@@ -0,0 +1,68 @@
+/* tracker.h — HTTP and UDP tracker clients (BEP-3/BEP-23, BEP-15).
+ *
+ * Split into pure codec (URL building, bencode response parsing, UDP packet
+ * encode/decode — all unit-testable without a socket) and thin blocking fetch
+ * helpers used by the swarm app. HTTPS/TLS is deferred to a later transport
+ * backend; the built-ins in this phase are plaintext HTTP and UDP.
+ */
+#ifndef NAUT_TRACKER_H
+#define NAUT_TRACKER_H
+
+#include "naut/common.h"
+
+/* IPv4 compact peer (BEP-23). IPv6 (BEP-7) is a later addition. */
+typedef struct { uint8_t ip[4]; uint16_t port; } naut_peer_addr;
+
+typedef enum {
+    NAUT_TEV_NONE = 0, NAUT_TEV_COMPLETED = 1, NAUT_TEV_STARTED = 2, NAUT_TEV_STOPPED = 3
+} naut_tracker_event;   /* values match BEP-15 UDP event codes */
+
+typedef struct {
+    uint8_t  info_hash[20];
+    uint8_t  peer_id[20];
+    uint16_t port;
+    uint64_t uploaded, downloaded, left;
+    naut_tracker_event event;
+    int32_t  numwant;       /* -1 for default */
+    uint32_t key;
+} naut_announce_req;
+
+typedef struct {
+    int32_t         interval;
+    int32_t         seeders, leechers;   /* -1 if absent */
+    naut_peer_addr *peers;
+    size_t          num_peers;
+    char           *failure;             /* tracker "failure reason", or NULL */
+} naut_tracker_response;
+
+void naut_tracker_response_free(naut_tracker_response *r);
+
+/* --- HTTP --- */
+/* Build the full announce GET URL (base?...params) with percent-encoded binary
+ * info_hash/peer_id. Returns bytes written (excl NUL) or 0 on overflow. */
+size_t naut_tracker_http_url(const char *base, const naut_announce_req *req,
+                             char *out, size_t outsz);
+/* Parse a bencoded HTTP tracker response body (compact or dict peer list). */
+naut_err naut_tracker_parse_http(const uint8_t *body, size_t len,
+                                 naut_tracker_response *out);
+
+/* --- UDP (BEP-15): pure packet codec --- */
+#define NAUT_UDP_CONNECT_REQ_LEN   16
+#define NAUT_UDP_ANNOUNCE_REQ_LEN  98
+void     naut_udp_build_connect(uint8_t out[16], uint32_t txid);
+naut_err naut_udp_parse_connect(const uint8_t *in, size_t len, uint32_t txid,
+                                uint64_t *connection_id);
+void     naut_udp_build_announce(uint8_t out[98], uint64_t connection_id,
+                                 uint32_t txid, const naut_announce_req *req);
+naut_err naut_udp_parse_announce(const uint8_t *in, size_t len, uint32_t txid,
+                                 naut_tracker_response *out);
+
+/* --- live fetch helpers (blocking) --- */
+/* HTTP GET the announce URL; fills out. Only http:// (no TLS yet). */
+naut_err naut_tracker_announce_http(const char *url, naut_tracker_response *out);
+/* Full UDP connect+announce handshake against host:port. */
+naut_err naut_tracker_announce_udp(const char *host, uint16_t port,
+                                   const naut_announce_req *req,
+                                   naut_tracker_response *out);
+
+#endif /* NAUT_TRACKER_H */