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/metainfo.h

@@ -0,0 +1,68 @@
+/* metainfo.h — .torrent and magnet parsing (v1 / v2 / hybrid).
+ *
+ * The info-hash is computed over the *raw* bytes of the `info` dictionary as
+ * they appear in the file (bencode preserves that span), never by re-encoding:
+ *   v1 info-hash = SHA-1  (info-bytes)
+ *   v2 info-hash = SHA-256(info-bytes)
+ * A hybrid torrent carries both and so joins both the v1 and v2 swarms.
+ *
+ * Phase 2 parses v1 fully (name, piece length, file list, the 20-byte piece
+ * hash table) and computes the v2 info-hash + version for hybrid/v2 files; the
+ * full v2 file-tree / piece-layer plumbing is wired in Phase 3 (storage).
+ */
+#ifndef NAUT_METAINFO_H
+#define NAUT_METAINFO_H
+
+#include "naut/common.h"
+#include "naut/hash.h"
+
+typedef struct {
+    char   *path;     /* '/'-joined, NUL-terminated */
+    int64_t length;
+} naut_file;
+
+typedef struct naut_metainfo {
+    bool    has_v1, has_v2;
+    uint8_t infohash_v1[NAUT_SHA1_LEN];
+    uint8_t infohash_v2[NAUT_SHA256_LEN];
+
+    char   *name;
+    int64_t piece_length;
+    int64_t total_length;
+
+    /* v1 piece hash table: num_pieces * 20 bytes (owned copy) */
+    uint32_t       num_pieces;
+    const uint8_t *piece_hashes;
+
+    naut_file *files;     size_t num_files;
+    char     **trackers;  size_t num_trackers;   /* announce + announce-list, flattened */
+
+    /* internals kept alive so piece_hashes/name stay valid */
+    void *_owned;
+} naut_metainfo;
+
+naut_err naut_metainfo_parse(const uint8_t *data, size_t len, naut_metainfo *out);
+/* Parse a raw bencoded info dictionary obtained through BEP-9. Optional
+ * tracker URLs are copied into the resulting metainfo. */
+naut_err naut_metainfo_parse_info(const uint8_t *info, size_t info_len,
+                                  const char *const *trackers,
+                                  size_t num_trackers,
+                                  naut_metainfo *out);
+void     naut_metainfo_free(naut_metainfo *mi);
+
+/* hex of the v1 info-hash (41 bytes incl NUL) — for logging/RPC. */
+void naut_infohash_v1_hex(const naut_metainfo *mi, char out[41]);
+
+/* --- magnet links -------------------------------------------------------- */
+typedef struct {
+    bool    has_v1, has_v2;
+    uint8_t infohash_v1[NAUT_SHA1_LEN];
+    uint8_t infohash_v2[NAUT_SHA256_LEN];
+    char   *name;                              /* dn (display name), may be NULL */
+    char  **trackers; size_t num_trackers;     /* tr= params */
+} naut_magnet;
+
+naut_err naut_magnet_parse(const char *uri, naut_magnet *out);
+void     naut_magnet_free(naut_magnet *m);
+
+#endif /* NAUT_METAINFO_H */