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>

tests/integration/dht_fixture.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""Minimal BEP-5 get_peers responder for the trackerless magnet gate."""
+import socket
+import struct
+import sys
+
+peer_port = int(sys.argv[1])
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+sock.bind(("127.0.0.1", 0))
+print("DHT_PORT %d" % sock.getsockname()[1], flush=True)
+
+node_id = bytes(range(20))
+compact_peer = socket.inet_aton("127.0.0.1") + struct.pack("!H", peer_port)
+
+while True:
+    packet, address = sock.recvfrom(65535)
+    marker = b"1:t"
+    start = packet.find(marker)
+    if start < 0:
+        continue
+    length_start = start + len(marker)
+    colon = packet.find(b":", length_start)
+    if colon < 0:
+        continue
+    try:
+        tx_len = int(packet[length_start:colon])
+    except ValueError:
+        continue
+    tx = packet[colon + 1:colon + 1 + tx_len]
+    if len(tx) != tx_len:
+        continue
+    response = (
+        b"d1:rd2:id20:" + node_id +
+        b"6:valuesl6:" + compact_peer +
+        b"ee1:t" + str(tx_len).encode() + b":" + tx +
+        b"1:y1:re"
+    )
+    sock.sendto(response, address)