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/seeder.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+"""Seed a .torrent with libtorrent on 127.0.0.1 and print the listen port.
+Usage: seeder.py <file.torrent> <save_path>
+Runs until killed. Prints 'PORT <n>' once it is seeding."""
+import libtorrent as lt, os, sys, time
+
+enc_policy = 0 if os.environ.get("NAUT_FORCE_MSE") == "1" else 1
+
+torrent, save_path = sys.argv[1], sys.argv[2]
+ses = lt.session({
+    "listen_interfaces": "127.0.0.1:0",
+    "unchoke_slots_limit": 64,    # unchoke leechers fast
+    "in_enc_policy": enc_policy,
+    "out_enc_policy": enc_policy,
+    "alert_mask": lt.alert_category.all,
+})
+
+atp = lt.add_torrent_params()
+atp.ti = lt.torrent_info(torrent)
+atp.save_path = save_path
+atp.flags |= lt.torrent_flags.seed_mode       # data already present; skip recheck
+atp.flags &= ~lt.torrent_flags.paused         # must be active to accept peers
+atp.flags &= ~lt.torrent_flags.auto_managed   # don't let the queue re-pause it
+h = ses.add_torrent(atp)
+h.resume()
+
+deadline = time.time() + 30
+while not h.status().is_seeding and time.time() < deadline:
+    time.sleep(0.05)
+if not h.status().is_seeding:
+    print("ERROR: not seeding", flush=True); sys.exit(1)
+
+print("PORT %d" % ses.listen_port(), flush=True)
+while True:
+    for a in ses.pop_alerts():
+        print("ALERT %s: %s" % (type(a).__name__, a.message()), flush=True)
+    time.sleep(0.2)