Defensive (prior-art) disclosure from halfmarble LLC: heart-rate-recovery-gated rest-interval timing — the rest between exercise rounds ends when heart rate recovers to a personalized target (a percentage of age-derived maximum heart rate), bounded by minimum/maximum rest durations with a timeout fallback, rather than on a fixed timer, with earbud audio status updates whose frequency increases as the target nears. Includes named in-scope variations (recovery-slope, HRV/respiratory criteria, adaptive recovery percentage, non-striking interval training). Dedicated to the public domain under CC0 1.0; source code and the SteadyHeartBeat app/brand are governed separately and unaffected; the applicant's retained motor-control/bioenergetic biomarker methods are expressly carved out and not dedicated here.