One engine (src/relay/tcp) for the four TCP relays: connections live in a
fixed pool owned through PoolPtr, handshake, cipher state and buffers are
inline, timeouts are intrusive deadline lists on one timer and DNS answers
come back through generation-checked handles. A full pool refuses new
connections and pauses accepting until a slot frees.
ss-tunnel is the first user; its hand-written relay callbacks are gone.
tests/tunnel_interop.py covers every method against ss-server and
shadowsocks-rust.
