The bug that revealed the real problem The trigger was a bug report on timed breathing sessions. The app was confusing rounds with reps. Hold values that made sense for counted breathing made no sense for timed breathing. The concept of "20 breaths per minute" and "a round lasts 30 seconds" were being shoehorned into the same fields. And the long full-lungs retention in hormesis — a 60–120 second hold after rapid breathing — was being modeled as just a very long in-rep hold, which broke everything about how retentions should work. It wasn't a bug in the usual sense. It was a data model that couldn't express the domain it was trying to represent. The old model The engine was built around a Cycle : one inhale + optional holds + one exhale. Rounds were cyclesPerRound . The custom builder called cycles "rounds." Timed and counted modes were handled by a mode discriminator on each phase. Long retentions were just cycles with enormous hold values.…