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/make_fixture.py

@@ -0,0 +1,76 @@
+from __future__ import annotations
+
+import argparse
+import hashlib
+import os
+
+import libtorrent as lt
+
+from common import file_sha1, parse_size, write_manifest
+
+
+def deterministic_bytes(offset: int, size: int) -> bytes:
+    out = bytearray()
+    counter = offset // 32
+    while len(out) < size:
+        out.extend(hashlib.sha256(f"torrent-peer-interop:{counter}".encode()).digest())
+        counter += 1
+    return bytes(out[:size])
+
+
+def write_data(path: str, size: int) -> None:
+    with open(path, "wb") as f:
+        off = 0
+        while off < size:
+            n = min(1024 * 1024, size - off)
+            f.write(deterministic_bytes(off, n))
+            off += n
+
+
+def make_torrent(root: str, size: int, piece_size: int) -> str:
+    os.makedirs(root, exist_ok=True)
+    data_path = os.path.join(root, "data.bin")
+    torrent_path = os.path.join(root, "test.torrent")
+    write_data(data_path, size)
+
+    fs = lt.file_storage()
+    lt.add_files(fs, data_path)
+    t = lt.create_torrent(fs, piece_size=piece_size)
+    t.set_priv(True)
+    t.add_tracker("http://fixture:9/announce")
+    lt.set_piece_hashes(t, root)
+    with open(torrent_path, "wb") as f:
+        f.write(lt.bencode(t.generate()))
+    return torrent_path
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Create the offline interop fixture.")
+    ap.add_argument("--out", required=True)
+    ap.add_argument("--size", default="32M")
+    ap.add_argument("--piece-size", default="256K")
+    args = ap.parse_args()
+
+    size = parse_size(args.size)
+    piece_size = parse_size(args.piece_size)
+    torrent_path = make_torrent(args.out, size, piece_size)
+    data_path = os.path.join(args.out, "data.bin")
+
+    ti = lt.torrent_info(torrent_path)
+    write_manifest(
+        os.path.join(args.out, "manifest.json"),
+        name=ti.name(),
+        size=size,
+        piece_size=piece_size,
+        num_pieces=ti.num_pieces(),
+        sha1=file_sha1(data_path),
+        announce="http://fixture:9/announce",
+        torrent=os.path.basename(torrent_path),
+        data=os.path.basename(data_path),
+    )
+    print(f"fixture ready: {size} bytes, {ti.num_pieces()} pieces", flush=True)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())