Returns a tidy frame shaped for an external validator (the LLMRcontent
package's content-validation entry point): one row per labeled unit with the
aligned id, prediction, and gold. This is the hand-off frame; it does
not call LLMRcontent. Pass the result to LLMRcontent's validator (for
example LLMRcontent::validate_against_gold(frame)) for the heavier
diagnostics (confusion matrix, per-class precision/recall, calibration
curves) that are out of scope for this thin local estimator.
