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/udp_tracker.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+"""Minimal BEP-15 UDP tracker for the Phase 4 integration test."""
+import socket
+import struct
+import sys
+
+peer_port = int(sys.argv[1])
+connection_id = 0x0102030405060708
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+sock.bind(("127.0.0.1", 0))
+print("PORT %d" % sock.getsockname()[1], flush=True)
+
+while True:
+    packet, addr = sock.recvfrom(2048)
+    if len(packet) >= 16 and packet[8:12] == b"\x00\x00\x00\x00":
+        sock.sendto(struct.pack("!IIQ", 0, struct.unpack("!I", packet[12:16])[0],
+                                connection_id), addr)
+        continue
+    if len(packet) >= 98 and packet[8:12] == b"\x00\x00\x00\x01":
+        txid = struct.unpack("!I", packet[12:16])[0]
+        response = (
+            struct.pack("!IIIII", 1, txid, 1800, 0, 1)
+            + socket.inet_aton("127.0.0.1")
+            + struct.pack("!H", peer_port)
+        )
+        sock.sendto(response, addr)
+        print("REQUEST announce", flush=True)