Fit a monotone calibration curve to this reliability histogram using weighted Pool Adjacent Violators. Merge ReliabilityStat snapshots before fitting to retain each bin's evidence across shards.

Empty bins inherit the preceding fitted pool, or the first pool for leading empty bins. An empty histogram yields zero probabilities. Fitting uses O(numBins) time and memory; IsotonicCalibratorResult.calibrate interpolates the fitted knots.