Function softplus 

pub fn softplus(x: f64) -> f64

Numerically stable softplus: ln(1 + exp(x)).

For |x| > 50, uses asymptotic approximation:

• x > 50: ln(1 + exp(x)) ≈ x (error < 1e-22)
• x < -50: ln(1 + exp(x)) ≈ exp(x) ≈ 0 (return 0)

The transition at |x| = 50 is mathematically smooth to within crate::Wave precision. This is the standard numerically-stable implementation used throughout the Koren triode/pentode equations.