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/common.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+import hashlib
+import json
+import os
+import socket
+import time
+import sys
+
+ROOT = os.environ.get("TORRENT_PEER_ROOT", "/work")
+sys.path.insert(0, os.path.join(ROOT, "harness"))
+
+from torrent_meta import Metadata, load_metadata  # noqa: E402
+
+
+def parse_size(text: str) -> int:
+    s = text.strip().upper()
+    mult = 1
+    if s[-1:] in ("K", "M", "G"):
+        mult = {"K": 1024, "M": 1024**2, "G": 1024**3}[s[-1]]
+        s = s[:-1]
+    return int(s) * mult
+
+
+def file_sha1(path: str) -> str:
+    h = hashlib.sha1()
+    with open(path, "rb") as f:
+        for chunk in iter(lambda: f.read(1024 * 1024), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def write_manifest(path: str, **items) -> None:
+    tmp = path + ".tmp"
+    with open(tmp, "w", encoding="utf-8") as f:
+        json.dump(items, f, indent=2, sort_keys=True)
+        f.write("\n")
+    os.replace(tmp, path)
+
+
+def wait_for_tcp(host: str, port: int, timeout: float = 30.0) -> None:
+    deadline = time.time() + timeout
+    last_error = None
+    while time.time() < deadline:
+        try:
+            with socket.create_connection((host, port), timeout=1.0):
+                return
+        except OSError as exc:
+            last_error = exc
+            time.sleep(0.25)
+    raise TimeoutError(f"timed out waiting for {host}:{port}: {last_error}")
+
+
+def resolve_ipv4(host: str) -> str:
+    infos = socket.getaddrinfo(host, None, socket.AF_INET, socket.SOCK_STREAM)
+    if not infos:
+        raise OSError(f"no IPv4 address for {host}")
+    return infos[0][4][0]
+
+
+def touch_ready(path: str = "/tmp/seed-ready") -> None:
+    with open(path, "w", encoding="utf-8") as f:
+        f.write("ready\n")