Function bdf1_step 

pub fn bdf1_step<F: Fn(f64, f64) -> f64>(t0: f64, y0: f64, h: f64, f: F) -> f64

One BDF1 (backward / implicit Euler) step: solve y₁ = y₀ + h·f(t₁, y₁) by Newton iteration. L-stable — the workhorse for stiff systems where explicit Euler/RK would require an impractically tiny h.