The previous implementation skipped empty fields, which meant targets
differing only in which fields were populated could produce the same
string (e.g. {UserId:"A"} and {DomainId:"A"} both gave "A"). This
caused key collisions when the string was used in storage index keys.
Keep Status as a plain integer on the wire so that OpenAPI/swaggo
can generate correct enum definitions without needing to handle
the custom string encoding. The String() method is preserved for
logging and debugging.
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.
