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>

src/platform/system.c

@@ -0,0 +1,19 @@
+#include "naut/system.h"
+
+#include <sched.h>
+#include <unistd.h>
+
+naut_err naut_pin_current_thread(int cpu) {
+    int online = naut_online_cpus();
+    if (cpu < 0 || cpu >= online) return NAUT_ERR_RANGE;
+    cpu_set_t set;
+    CPU_ZERO(&set);
+    CPU_SET((unsigned)cpu, &set);
+    return sched_setaffinity(0, sizeof set, &set) == 0
+        ? NAUT_OK : NAUT_ERR_IO;
+}
+
+int naut_online_cpus(void) {
+    long count = sysconf(_SC_NPROCESSORS_ONLN);
+    return count > 0 && count <= INT32_MAX ? (int)count : 1;
+}