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/rc4.h

@@ -0,0 +1,21 @@
+/* rc4.h — ARC4 stream cipher for MSE/PE (BitTorrent message-stream encryption).
+ *
+ * MSE keys RC4 from a SHA-1 of the negotiated DH secret and *discards the first
+ * 1024 keystream bytes* before use (the well-known RC4 keystream-bias defense),
+ * so naut_rc4_init takes a drop count. This is obfuscation, not strong crypto —
+ * its job is firewall/ISP evasion, and the engineering concern here is that it
+ * costs real CPU at 10 GbE (see the throughput budget), hence it lives behind a
+ * tight, in-place API.
+ */
+#ifndef NAUT_RC4_H
+#define NAUT_RC4_H
+
+#include "naut/common.h"
+
+typedef struct { uint8_t s[256]; uint8_t i, j; } naut_rc4;
+
+void naut_rc4_init(naut_rc4 *c, const void *key, size_t keylen, size_t drop);
+/* XOR keystream into buf in place (encrypt == decrypt). */
+void naut_rc4_xor(naut_rc4 *c, void *buf, size_t len);
+
+#endif /* NAUT_RC4_H */