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/echo_client.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Exercise the io_uring fixed-buffer/SEND_ZC echo path with exact data."""
+import socket
+import sys
+import threading
+import time
+
+port = int(sys.argv[1])
+total = int(sys.argv[2]) if len(sys.argv) > 2 else 64 * 1024 * 1024
+connections = int(sys.argv[3]) if len(sys.argv) > 3 else 1
+chunk = bytes((i * 31 + 7) & 0xFF for i in range(128 * 1024))
+
+start = time.monotonic()
+errors = []
+
+def connection_worker(connection_bytes):
+    sock = socket.create_connection(("127.0.0.1", port), timeout=5)
+    send_error = []
+
+    def sender():
+        try:
+            sent = 0
+            while sent < connection_bytes:
+                payload = chunk[:min(len(chunk), connection_bytes - sent)]
+                sock.sendall(payload)
+                sent += len(payload)
+            sock.shutdown(socket.SHUT_WR)
+        except Exception as error:
+            send_error.append(error)
+
+    thread = threading.Thread(target=sender)
+    thread.start()
+    try:
+        done = 0
+        while done < connection_bytes:
+            part = sock.recv(min(1024 * 1024, connection_bytes - done))
+            if not part:
+                raise RuntimeError("echo server closed early")
+            offset = 0
+            while offset < len(part):
+                pattern_offset = (done + offset) % len(chunk)
+                count = min(len(part) - offset, len(chunk) - pattern_offset)
+                if part[offset:offset + count] != chunk[pattern_offset:pattern_offset + count]:
+                    raise RuntimeError("echo content mismatch")
+                offset += count
+            done += len(part)
+        thread.join()
+        if send_error:
+            raise send_error[0]
+    except Exception as error:
+        errors.append(error)
+    finally:
+        sock.close()
+
+per_connection = total // connections
+workers = [
+    threading.Thread(target=connection_worker, args=(per_connection,))
+    for _ in range(connections)
+]
+for worker in workers:
+    worker.start()
+for worker in workers:
+    worker.join()
+if errors:
+    raise errors[0]
+elapsed = time.monotonic() - start
+print("ECHO %.2f Gbit/s (%d connections)" %
+      (per_connection * connections * 8 / elapsed / 1e9, connections),
+      flush=True)