Constant MIN_ALPHA 

pub const MIN_ALPHA: f64 = 0.1;

The smallest decay exponent α a pow-3 fit may report and still be trusted as an asymptote — the shape half of the extrapolation fail-safe.

As α → 0 the model c − a·t^(−α) degenerates: t^(−α) → 1, so the curve is effectively a straight line with no deceleration, and its fitted c is an unbounded straight-line projection (a linear y = 0.5·t fits with α ≈ 0.006, c ≈ 241 — eighty times the largest value observed), not a plateau the points approach. A curve that flat is still rising, not decelerating toward a limit, so fit_pow3 rejects it: a model you cannot build is not evidence to freeze on. The floor sits in the empirical gap between the mirages of the benchmark’s still-rising families (α ≤ 0.09) and its genuine decelerating plateaus (α ≥ 0.16, including a truly flat curve, which fits with a ≈ 0 and a well-determined α ≈ 0.8).