lightning: watchdog only recycles on real backlog, not normal idle

The zombie-listener watchdog (bd1ece38) treated "waitanyinvoice blocked > 5min
while cl is alive" as a stall — but that's NORMAL during quiet periods, so it
false-fired and needlessly recycled the cl listen socket every few minutes
(~26x in 6h on 2026-06-07). Receives still worked, but it was churn + noise and
would mask a genuine zombie.

Fix: a blocked long-poll is only a zombie if cl actually has a paid invoice the
listener isn't picking up. When blocked past the threshold, probe cl on the MAIN
socket with waitanyinvoice(pay_index, 2s server-side timeout): if it returns an
invoice there's a real backlog the wedged listener missed -> recycle; if it
times out (CLN code 904 = nothing waiting) the listener is just idle and healthy
-> leave it alone. Only recycle on a confirmed backlog.

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

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