Preregistration: Flow-Lenia dose-response atlas v3

Prospective final construction repair of the Flow-Lenia third-substrate dose-response atlas. V1 and v2 stopped before any confirmatory treatment transition. V3 uses a zero-lower-bounded centered pair with deterministic contrast calibration and prospective direction-retention gates. The frozen cohort, six-dose ladder, raw endpoints, bootstrap analysis, decision thresholds, replay, and claim boundary remain unchanged.