Each listener binds to its own port with an independent chain.
The "pool" keyword in a chain appends a random alive proxy from
the shared pool; multiple pool entries = multiple hops.
:1080 -> Tor only (0 pool hops)
:1081 -> Tor + 1 pool proxy
:1082 -> Tor + 2 pool proxies
Shared resources (ProxyPool, Tor, metrics, semaphore, API) are
reused across listeners. FirstHopPool is shared per unique first
hop. Backward compatible: old listen/chain format still works.
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