API:
https://swimmingcalculator.steven-mcintyre1903.workers.devHow Estimates Are Calculated
We fit a power-law t = A·d^B per stroke using any available PBs (after a WR-based pacing correction). If data is sparse,
we infer B from world-record pacing and (if needed) synthesize a base point from another stroke at the same distance
via world-record ratios. Output shown as m:ss:cc.