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_tracker_swarm.sh

@@ -0,0 +1,129 @@
+#!/usr/bin/env bash
+# Phase 4 tracker gate: discover a real libtorrent seed from a localhost HTTP
+# tracker, then complete the download without explicit peer arguments.
+set -u
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+SWARM="${1:-$ROOT/build/naut_swarm}"
+MODE="${2:-http}"
+SEEDER="$ROOT/tests/integration/seeder.py"
+if [ "$MODE" = "udp" ]; then
+  TRACKER="$ROOT/tests/integration/udp_tracker.py"
+else
+  TRACKER="$ROOT/tests/integration/http_tracker.py"
+fi
+SOURCE_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_tracker_swarm.XXXXXX)"
+seed_log="$(mktemp /tmp/naut_tracker_seed.XXXXXX)"
+tracker_log="$(mktemp /tmp/naut_http_tracker.XXXXXX)"
+swarm_log="$(mktemp /tmp/naut_tracker_client.XXXXXX)"
+tor="$(mktemp /tmp/naut_tracker.XXXXXX.torrent)"
+seed=""
+tracker=""
+
+cleanup() {
+  [ -n "$seed" ] && kill "$seed" 2>/dev/null || true
+  [ -n "$tracker" ] && kill "$tracker" 2>/dev/null || true
+  rm -rf "$out" "$seed_log" "$tracker_log" "$swarm_log" "$tor"
+}
+trap cleanup EXIT
+
+python3 "$SEEDER" "$SOURCE_TOR" "$DATA" > "$seed_log" 2>&1 &
+seed=$!
+seed_port=""
+for _ in $(seq 1 100); do
+  seed_port="$(grep -oP 'PORT \K[0-9]+' "$seed_log" 2>/dev/null || true)"
+  [ -n "$seed_port" ] && [ "$seed_port" != 0 ] && break
+  sleep 0.1
+done
+if [ -z "$seed_port" ] || [ "$seed_port" = 0 ]; then
+  echo "FAIL: seeder did not start"
+  cat "$seed_log"
+  exit 1
+fi
+
+python3 "$TRACKER" "$seed_port" > "$tracker_log" 2>&1 &
+tracker=$!
+tracker_port=""
+for _ in $(seq 1 100); do
+  tracker_port="$(grep -oP 'PORT \K[0-9]+' "$tracker_log" 2>/dev/null || true)"
+  [ -n "$tracker_port" ] && break
+  sleep 0.05
+done
+if [ -z "$tracker_port" ]; then
+  echo "FAIL: tracker did not start"
+  cat "$tracker_log"
+  exit 1
+fi
+
+# Replace only the top-level dictionary. The raw info dictionary is copied
+# byte-for-byte so its SHA-1 info hash remains unchanged.
+if [ "$MODE" = "udp" ]; then
+  announce="udp://127.0.0.1:$tracker_port/announce"
+else
+  announce="http://127.0.0.1:$tracker_port/announce"
+fi
+python3 - "$SOURCE_TOR" "$tor" "$announce" <<'PY'
+import sys
+
+
+def skip(data, pos):
+    token = data[pos]
+    if token == ord("i"):
+        return data.index(b"e", pos) + 1
+    if token in (ord("l"), ord("d")):
+        pos += 1
+        while data[pos] != ord("e"):
+            pos = skip(data, pos)
+            if token == ord("d"):
+                pos = skip(data, pos)
+        return pos + 1
+    colon = data.index(b":", pos)
+    size = int(data[pos:colon])
+    return colon + 1 + size
+
+
+source, target, announce = sys.argv[1], sys.argv[2], sys.argv[3].encode()
+data = open(source, "rb").read()
+pos = 1
+raw_info = None
+while data[pos] != ord("e"):
+    colon = data.index(b":", pos)
+    key_len = int(data[pos:colon])
+    key_start = colon + 1
+    key = data[key_start:key_start + key_len]
+    pos = key_start + key_len
+    value_start = pos
+    pos = skip(data, pos)
+    if key == b"info":
+        raw_info = data[value_start:pos]
+assert raw_info is not None
+rewritten = (
+    b"d8:announce" + str(len(announce)).encode() + b":" + announce
+    + b"4:info" + raw_info + b"e"
+)
+open(target, "wb").write(rewritten)
+PY
+
+if ! timeout 30 "$SWARM" "$tor" "$out" > "$swarm_log" 2>&1; then
+  echo "FAIL: tracker-discovered swarm did not complete"
+  cat "$swarm_log" "$tracker_log"
+  exit 1
+fi
+if ! diff -r "$out/single.bin" "$DATA/single.bin" >/dev/null; then
+  echo "FAIL: content mismatch"
+  exit 1
+fi
+if ! grep -q "REQUEST" "$tracker_log"; then
+  echo "FAIL: tracker received no announce"
+  exit 1
+fi
+
+echo "PASS: $MODE tracker discovery produced byte-identical output"