Function sustained_miss 

pub fn sustained_miss(
    frame_secs: impl Iterator<Item = f32>,
    budget_secs: f32,
) -> bool

Whether a frame-time series shows a sustained miss of the display budget, which is the one condition that demotes Tier::Rich to Tier::Floor (ADR-0045).

Pure and total: a function of the series and the budget with no clock, no state and no allocation, so the policy is unit-testable against injected series (which is how the spike-versus-overload distinction above is checked rather than asserted). frame_secs is the rolling history in seconds, the unit FrameStats::samples yields; order does not matter, only the counts.

Says false for a non-positive or non-finite budget, and for a series shorter than MIN_SAMPLES — the safe direction, since a wrong true costs the user the rich tier for the rest of the session and a wrong false costs nothing but another second of measurement.