Add FirstHopPool that maintains a deque of pre-established TCP
connections to chain[0]. Connections idle beyond pool_max_idle are
evicted; a background task refills to pool_size. build_chain() tries
the pool first, falls back to open_connection. Enabled with
pool_size > 0 in config. Only pools the TCP handshake -- SOCKS/HTTP
tunnels are consumed, not returned.
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