Calibrate Scale Region

Set or update a region's calibration.

Server computes scale_ratio from the embedded CalibrationData via the existing compute_scale_ratio and recomputes only this region's measurements, atomically (spec §5.3). Mirrors UpdatePageCalibrationRequest.

Returned by calibrate / edit / delete: the region (omitted on delete is out of scope — delete returns only the count) plus how many measurements were recomputed in the same transaction.