ln/lightning: watchdog recycles zombie receive-listener socket

Port of master bd1ece3 to staging. When cl restarts (e.g. autoheal after a
healthcheck timeout), the app's waitanyinvoice long-poll can hang on the now-dead
listen socket: listenerActive stays true forever, nothing restarts it, and
incoming lightning payments silently stop crediting until a manual app restart.
The per-call RPC timeout can't catch this — waitanyinvoice is meant to block.

ensureListenerAlive (run every 120s) now also detects a stalled listener — active
but blocked in waitanyinvoice for >5min — probes cl via the main socket (getinfo),
and only if cl is actually alive treats the listen socket as a zombie: bumps a
listenerEpoch, calls lnListen.reset() (new: drops the proxy socket so the next
call reconnects), and re-arms. The epoch guard makes the wedged invocation bow out
when its waitanyinvoice finally rejects, so we never run two listeners or
double-credit.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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