An unreadable or unparsable known_hosts file was swallowed with
'let _', silently downgrading to prompt-and-accept without telling the
user why their configuration was ignored: warn naming the file, then
continue. And when the pinned Launchpad fingerprint matches, a
DIFFERENT key recorded for that host in known_hosts was silently
bypassed: warn about the stale entry (diagnostic only — the published
fingerprint stays authoritative).