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/run_magnet_dht.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# Phase 5 gate: trackerless magnet -> DHT peer -> ut_metadata -> verified data.
+set -u
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+SWARM="${1:-$ROOT/build/naut_swarm}"
+SEEDER="$ROOT/tests/integration/seeder.py"
+DHT="$ROOT/tests/integration/dht_fixture.py"
+TORRENT="$ROOT/tests/fixtures/single_v1.torrent"
+DATA="$ROOT/tests/fixtures/data"
+MAGNET="magnet:?xt=urn:btih:7f2555dfd18ba1c4a024264e939d7a5f8b25311b"
+
+python3 -c 'import libtorrent' 2>/dev/null || {
+  echo "SKIP: python libtorrent not available"; exit 77;
+}
+[ -x "$SWARM" ] || { echo "FAIL: $SWARM not built"; exit 1; }
+
+out="$(mktemp -d /tmp/naut_magnet.XXXXXX)"
+seed_log="$(mktemp /tmp/naut_magnet_seed.XXXXXX)"
+dht_log="$(mktemp /tmp/naut_magnet_dht.XXXXXX)"
+client_log="$(mktemp /tmp/naut_magnet_client.XXXXXX)"
+cleanup() {
+  [ -n "${seed:-}" ] && kill "$seed" 2>/dev/null || true
+  [ -n "${dht_pid:-}" ] && kill "$dht_pid" 2>/dev/null || true
+  rm -rf "$out" "$seed_log" "$dht_log" "$client_log"
+}
+trap cleanup EXIT
+
+python3 "$SEEDER" "$TORRENT" "$DATA" >"$seed_log" 2>&1 &
+seed=$!
+peer_port=""
+for _ in $(seq 1 100); do
+  peer_port=$(grep -oP 'PORT \K[0-9]+' "$seed_log" 2>/dev/null)
+  [ -n "$peer_port" ] && break
+  sleep 0.1
+done
+[ -n "$peer_port" ] || {
+  echo "FAIL: seeder did not start"; cat "$seed_log"; exit 1;
+}
+
+python3 "$DHT" "$peer_port" >"$dht_log" 2>&1 &
+dht_pid=$!
+dht_port=""
+for _ in $(seq 1 100); do
+  dht_port=$(grep -oP 'DHT_PORT \K[0-9]+' "$dht_log" 2>/dev/null)
+  [ -n "$dht_port" ] && break
+  sleep 0.1
+done
+[ -n "$dht_port" ] || {
+  echo "FAIL: DHT fixture did not start"; cat "$dht_log"; exit 1;
+}
+
+if ! NAUT_DHT_BOOTSTRAP="127.0.0.1:$dht_port" \
+    timeout 30 "$SWARM" "$MAGNET" "$out" >"$client_log" 2>&1; then
+  echo "FAIL: trackerless magnet download did not complete"
+  cat "$client_log"
+  cat "$seed_log"
+  exit 1
+fi
+grep -q "magnet metadata verified" "$client_log" || {
+  echo "FAIL: metadata was not verified"; cat "$client_log"; exit 1;
+}
+diff -r "$out/single.bin" "$DATA/single.bin" >/dev/null || {
+  echo "FAIL: magnet content mismatch"; exit 1;
+}
+echo "PASS: DHT-only magnet completed with verified metadata and content"