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/Dockerfile

@@ -0,0 +1,32 @@
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    aria2 \
+    build-essential \
+    ca-certificates \
+    cmake \
+    deluge-console \
+    deluged \
+    procps \
+    python3 \
+    python3-libtorrent \
+    qbittorrent-nox \
+    rtorrent \
+    transmission-cli \
+    transmission-daemon \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /work
+
+COPY CMakeLists.txt /work/CMakeLists.txt
+COPY include /work/include
+COPY src /work/src
+COPY harness /work/harness
+COPY interop /work/interop
+
+RUN cmake -S /work -B /work/build -DCMAKE_BUILD_TYPE=Release -DPEER_NATIVE=OFF \
+    && cmake --build /work/build
+
+ENV PYTHONPATH=/work/harness:/work/interop