Constant DECAY_MIN 

const DECAY_MIN: f64 = 0.05;

The range a decay time constant is allowed to take, in years.

A bound, not a preference. As λ → ∞ the slope loading tends to one and the hump loading to zero, so β₀, β₁ and β₂ trade off along a direction that barely moves the curve — the parameters stop being identified. A fit then wanders up that ridge indefinitely: measured on a government universe, one ran to β₀ = 921, β₁ = −921, λ = 1420 years and was still creeping at the iteration cap. A decay of fourteen centuries is not a yield-curve shape.

Bounding it makes the model identified. Storing the unconstrained variable and squashing it here keeps the parameter free for the solver to step, and the bound holds by construction rather than by a clamp the solver has to be trusted to respect — the same argument that put the decay in logarithms, one step further.