Make StatusUnknown the zero value (0) so an uninitialized CheckState
reads as "no signal yet" rather than as healthy. Push StatusOK and
StatusInfo to negative values so the natural int ordering matches
severity ordering: aggregators can simply take max() to compute the
worst status, and Unknown correctly sits above OK/Info but below Warn.
Status now (un)marshals as its string name ("OK", "WARN", ...) so the
wire format is stable across any future renumbering. UnmarshalJSON
still accepts raw ints for backward compatibility with older snapshots
and clients.
RegisterChecker and RegisterObservationProvider previously overwrote
existing entries silently, which lets a misconfigured plugin shadow a
built-in (or another plugin) without any signal. Refuse the duplicate
and keep the existing entry instead.
RegisterExternalizableChecker now performs the dedup check before
appending the "endpoint" AdminOpt, so a rejected re-registration no
longer mutates the live definition.
