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/net.h

@@ -0,0 +1,26 @@
+/* net.h — socket setup (the kernel-facing seam, part 1).
+ *
+ * All socket option choices that matter for 10 GbE live here: SO_REUSEPORT so
+ * each reactor owns a listen queue and the kernel shards inbound peers across
+ * cores, TCP_NODELAY (BitTorrent is latency-sensitive on small control msgs),
+ * and large socket buffers so the BDP fits.
+ */
+#ifndef NAUT_NET_H
+#define NAUT_NET_H
+
+#include "naut/common.h"
+#include <netinet/in.h>
+
+/* Create a TCP listener bound to `port`. With reuseport=true, multiple reactors
+ * may each create one on the same port and the kernel load-balances accepts. */
+int naut_net_listen(uint16_t port, int backlog, bool reuseport);
+
+/* Tune an accepted/connected peer socket for throughput. */
+void naut_net_tune_peer(int fd);
+
+/* Set send/recv socket buffer sizes (bytes); 0 leaves the kernel default. */
+void naut_net_set_bufsizes(int fd, int sndbuf, int rcvbuf);
+
+int naut_net_set_nonblock(int fd, bool on);
+
+#endif /* NAUT_NET_H */