Initial commit: multi-peer torrent download engine

Reactor/loop-pool engine with TCP/µTP/MSE transports, per-connection
pipelining, priority-driven piece selection with endgame, and the Python
FFI test harness.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

interop/seed_aria2.py

@@ -0,0 +1,66 @@
+from __future__ import annotations
+
+import argparse
+import signal
+import subprocess
+import time
+
+from common import touch_ready, wait_for_tcp
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Seed a fixture with aria2.")
+    ap.add_argument("--torrent", required=True)
+    ap.add_argument("--data", required=True)
+    ap.add_argument("--port", type=int, required=True)
+    args = ap.parse_args()
+
+    cmd = [
+        "aria2c",
+        "--dir", args.data,
+        "--seed-time=1000000",
+        "--check-integrity=true",
+        "--allow-overwrite=false",
+        "--auto-file-renaming=false",
+        "--enable-dht=false",
+        "--enable-dht6=false",
+        "--enable-peer-exchange=false",
+        "--bt-enable-lpd=false",
+        "--listen-port", str(args.port),
+        "--dht-listen-port", str(args.port),
+        "--summary-interval=0",
+        args.torrent,
+    ]
+    proc = subprocess.Popen(cmd)
+    stop = False
+
+    def _stop(signum, frame):
+        nonlocal stop
+        stop = True
+        proc.terminate()
+
+    signal.signal(signal.SIGTERM, _stop)
+    signal.signal(signal.SIGINT, _stop)
+
+    try:
+        wait_for_tcp("127.0.0.1", args.port, timeout=45)
+        if proc.poll() is not None:
+            return proc.returncode or 1
+        # aria2 may still be checking files; the runner also has retry/timeout,
+        # so readiness here means the peer port is accepting connections.
+        touch_ready()
+        print(f"aria2 seeding on {args.port}", flush=True)
+        while not stop and proc.poll() is None:
+            time.sleep(1)
+        return proc.returncode or 0
+    finally:
+        if proc.poll() is None:
+            proc.terminate()
+            try:
+                proc.wait(timeout=10)
+            except subprocess.TimeoutExpired:
+                proc.kill()
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())