ln: tear down dead cl clients fully + recover invoice listener on socket death

Replace bare `client = null` with dropClient(): stop the clightning-client
library's internal reconnect loop (clear reconnectTimeout, neuter reconnect,
destroy the socket, drop listeners). A dead client was otherwise orphaned and
spun forever retrying `connect ENOENT`, spamming logs and leaking a socket+timer
on every cl restart. Guard the error handler on `c === client` so a stale
client's late error can't drop a newer, healthy one.

Add lnListen.onDrop so the invoice listener recycles immediately when its
long-poll socket dies (bump epoch, re-arm) instead of waiting up to 5 minutes for
the stall watchdog. Stop counting LightningUnavailableError toward the listener's
process.exit escalation, so a cl restart no longer makes the app self-exit — it
retries until cl returns.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

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