Loading-feedback tuning: the visibility-gate timings shared by every surface
that gates its visibility through createVisibilityGate, and the
indicator-selection thresholds a consumer reads to choose one loading
treatment over another.
Duration in ms above which an app-shell indicator should prefer a skeleton
over a spinner. A consumer compares an observed busy-envelope duration
(e.g. createLatencyProbe().lastDuration()) against this cutoff.