feat(factor): implement TruncFactor with cached evidence

EP truncation factor that operates on a diff variable. Stores its
outgoing message so the cavity computation produces the correct EP
message on each propagation. The first propagation caches the
evidence contribution (cdf-bounded probability) for log_evidence().

Promotes lib::cdf to pub(crate) so the factor can use it.

src/lib.rs

@@ -163,7 +163,7 @@ fn compute_margin(p_draw: f64, sd: f64) -> f64 {
     ppf(0.5 - p_draw / 2.0, 0.0, sd).abs()
 }
 
-fn cdf(x: f64, mu: f64, sigma: f64) -> f64 {
+pub(crate) fn cdf(x: f64, mu: f64, sigma: f64) -> f64 {
     let z = -(x - mu) / (sigma * SQRT_2);
 
     0.5 * erfc(z)