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_swarm.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+# Phase 4 interop gate: download one torrent concurrently from two independent
+# libtorrent seeds and require both peers to contribute blocks.
+set -u
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+SWARM="${1:-$ROOT/build/naut_swarm}"
+SEEDER="$ROOT/tests/integration/seeder.py"
+TOR="$ROOT/tests/fixtures/single_v1.torrent"
+DATA="$ROOT/tests/fixtures/data"
+
+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_swarm.XXXXXX)"
+log1="$(mktemp /tmp/naut_seed1.XXXXXX)"
+log2="$(mktemp /tmp/naut_seed2.XXXXXX)"
+slog="$(mktemp /tmp/naut_swarm_log.XXXXXX)"
+seed1=""
+seed2=""
+
+cleanup() {
+  [ -n "$seed1" ] && kill "$seed1" 2>/dev/null || true
+  [ -n "$seed2" ] && kill "$seed2" 2>/dev/null || true
+  rm -rf "$out" "$log1" "$log2" "$slog"
+}
+trap cleanup EXIT
+
+python3 "$SEEDER" "$TOR" "$DATA" > "$log1" 2>&1 &
+seed1=$!
+python3 "$SEEDER" "$TOR" "$DATA" > "$log2" 2>&1 &
+seed2=$!
+
+port1=""
+port2=""
+for _ in $(seq 1 100); do
+  port1="$(grep -oP 'PORT \K[0-9]+' "$log1" 2>/dev/null || true)"
+  port2="$(grep -oP 'PORT \K[0-9]+' "$log2" 2>/dev/null || true)"
+  [ -n "$port1" ] && [ "$port1" != 0 ] &&
+    [ -n "$port2" ] && [ "$port2" != 0 ] && break
+  sleep 0.1
+done
+if [ -z "$port1" ] || [ "$port1" = 0 ] ||
+   [ -z "$port2" ] || [ "$port2" = 0 ]; then
+  echo "FAIL: seeders did not start"
+  cat "$log1" "$log2"
+  exit 1
+fi
+
+if ! timeout 30 "$SWARM" "$TOR" "$out" \
+     "127.0.0.1:$port1" "127.0.0.1:$port2" > "$slog" 2>&1; then
+  echo "FAIL: swarm download did not complete"
+  cat "$slog"
+  exit 1
+fi
+if ! diff -r "$out/single.bin" "$DATA/single.bin" >/dev/null; then
+  echo "FAIL: content mismatch"
+  exit 1
+fi
+contributors="$(grep -Ec 'delivered [1-9][0-9]* blocks' "$slog" || true)"
+if [ "$contributors" -lt 2 ]; then
+  echo "FAIL: expected both peers to contribute"
+  cat "$slog"
+  exit 1
+fi
+
+echo "PASS: two-peer swarm produced byte-identical output"