Probit: P = 1/2 − erf(√2(μ−x)/(2σ))/2; P(μ)=1/2; slope@LD_50 = √2/(2√π·σ)

Sympy-exact symbolic witness of the probit dose-response canonical form and its LD_50 and slope anchors. Setup: x (log-dose), μ (log-LD_50), σ (log-dose standard deviation) — all positive symbols. Probit model: P(death | x) = Φ((x−μ)/σ)…