bench: capture T0 baseline; expose pi/tau accessors; fix div panic

- Promotes Gaussian::pi and Gaussian::tau to public so benches/gaussian.rs
  compiles, then captures baseline numbers for the T0 acceptance gate.
- Fixes the divide bench: g1/g2 panicked (g1 has lower precision than g2;
  cavity requires pi_num >= pi_den). Swapped to g2/g1 (well-defined).

Baseline on Apple M5 Pro:
  Batch::iteration  29.840 µs
  Gaussian::mul      1.568 ns   (vs ~220 ps for add/sub — hot path)
  Gaussian::div      1.572 ns

src/gaussian.rs

@@ -13,7 +13,7 @@ impl Gaussian {
         Gaussian { mu, sigma }
     }
 
-    fn pi(&self) -> f64 {
+    pub fn pi(&self) -> f64 {
         if self.sigma > 0.0 {
             self.sigma.powi(-2)
         } else {
@@ -21,7 +21,7 @@ impl Gaussian {
         }
     }
 
-    fn tau(&self) -> f64 {
+    pub fn tau(&self) -> f64 {
         if self.sigma > 0.0 {
             self.mu * self.pi()
         } else {