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/bencode.h

@@ -0,0 +1,77 @@
+/* bencode.h — bencode parser/encoder for .torrent, tracker, and DHT messages.
+ *
+ * The parser is zero-copy: strings are slices into the caller's buffer, never
+ * duplicated. Every parsed value also records its exact raw byte span, which is
+ * what lets us compute an info-hash over the *original* encoding of the `info`
+ * dict without re-serializing (re-serialization would risk a non-canonical
+ * byte sequence and a wrong hash). Container children live in an arena owned by
+ * the document, so the whole tree frees in one shot.
+ *
+ * Hardened for hostile input (it parses bytes straight off the wire): explicit
+ * depth and node-count limits, strict integer/string syntax, no unbounded
+ * recursion blow-up. This is a primary fuzz target.
+ */
+#ifndef NAUT_BENCODE_H
+#define NAUT_BENCODE_H
+
+#include "naut/common.h"
+
+typedef enum {
+    NAUT_BC_INT, NAUT_BC_STR, NAUT_BC_LIST, NAUT_BC_DICT
+} naut_bc_type;
+
+typedef struct naut_bc naut_bc;
+
+typedef struct {
+    const uint8_t *kp; size_t kn;   /* key slice (bencode dict keys are strings) */
+    const naut_bc  *val;
+} naut_bc_pair;
+
+struct naut_bc {
+    naut_bc_type type;
+    union {
+        int64_t i;
+        struct { const uint8_t *p; size_t n; } str;
+        struct { const naut_bc     *items; size_t count; } list;
+        struct { const naut_bc_pair *pairs; size_t count; } dict;
+    } v;
+    const uint8_t *raw;     /* exact encoding span of this value... */
+    size_t         raw_len; /* ...used for info-hash over the original bytes */
+};
+
+typedef struct naut_bc_doc naut_bc_doc;
+
+/* Parse the entire buffer as one bencode value. Rejects trailing garbage.
+ * On success *out owns the tree (free with naut_bc_free). */
+naut_err naut_bc_parse(const uint8_t *data, size_t len, naut_bc_doc **out);
+/* Parse one value from the start of a larger buffer. `consumed` receives the
+ * encoded value length; trailing bytes remain owned by the caller. */
+naut_err naut_bc_parse_prefix(const uint8_t *data, size_t len,
+                              naut_bc_doc **out, size_t *consumed);
+const naut_bc *naut_bc_root(const naut_bc_doc *doc);
+void naut_bc_free(naut_bc_doc *doc);
+
+/* Accessors (return NULL / defaults on type mismatch). */
+const naut_bc *naut_bc_dict_get(const naut_bc *d, const char *key);
+const naut_bc *naut_bc_list_at(const naut_bc *l, size_t i);
+bool naut_bc_get_int(const naut_bc *v, int64_t *out);
+bool naut_bc_get_str(const naut_bc *v, const uint8_t **p, size_t *n);
+/* true if a STR value equals the given C string exactly */
+bool naut_bc_str_eq(const naut_bc *v, const char *s);
+
+/* --- encoder (DHT/tracker/.torrent writing): append to a growable buffer --- */
+typedef struct {
+    uint8_t *buf; size_t len, cap;
+    naut_err err;
+} naut_bc_writer;
+
+void naut_bc_w_init(naut_bc_writer *w);
+void naut_bc_w_free(naut_bc_writer *w);
+void naut_bc_w_int(naut_bc_writer *w, int64_t v);
+void naut_bc_w_bytes(naut_bc_writer *w, const void *p, size_t n);
+void naut_bc_w_cstr(naut_bc_writer *w, const char *s);
+void naut_bc_w_list_begin(naut_bc_writer *w);
+void naut_bc_w_dict_begin(naut_bc_writer *w);
+void naut_bc_w_end(naut_bc_writer *w);  /* closes the current list/dict */
+
+#endif /* NAUT_BENCODE_H */