A successful Sudski registar API response: the decoded data plus the metadata
the service returns in X-* headers.
data is whatever Req produced — a decoded list/map for JSON, a raw string
otherwise (XML, or an empty body). Numeric headers are integer-parsed;
seconds_elapsed and timestamp stay raw strings (the timestamp is
timezone-less, so parsing is left to the caller).
Note: log_id appears here (integer-parsed, on success) and on
SudregEx.Error (verbatim, possibly a string, on a JSON error) — a deliberate
asymmetry between the success and error envelopes.