Constant REMAINING_FACTOR 

pub const REMAINING_FACTOR: f64 = 3.0;

The largest extrapolated improvement a pow-3 fit may predict, as a multiple of the improvement already observed — the magnitude half of the fail-safe.

A genuine plateau sits only a little above the last observed value; a mirage on a still-rising curve places its asymptote many times the observed range above it (the benchmark’s still-rising windows extrapolate to 10×–80× the range). fit_pow3 rejects a fit whose remaining rise c − y_max exceeds REMAINING_FACTOR · the observed rise y_max − y_min. 3 is chosen with headroom above every genuine early-window fit measured (the widest was ≈2.5×) yet far below every mirage, so it never rejects a plateau that the shape guard would have kept.