refactor(gaussian): switch to natural-parameter storage (pi, tau)

Mul and Div become two f64 adds/subs with no sqrt in the hot path.
mu() and sigma() are computed on demand from stored pi/tau.

Key implementation notes:
- exclude() returns N00 when var <= 0 to avoid inf/inf = NaN when
  two Gaussians have the same precision (ULP-level round-trip error
  from the pi→sigma accessor).
- Mul<f64> by 0.0 returns N00 (point mass at 0), matching old behavior.
- from_ms(0, 0) == N00 {pi:inf, tau:0}; from_ms(0, inf) == N_INF {pi:0, tau:0}.

Golden values in test_1vs1vs1_draw updated: nat-param arithmetic
rounds mu to 25.0 (was 24.999999) and shifts sigma by ~3e-7.
Both differences are bounded and validated against the original Python
reference values.

Part of T0 engine redesign.

src/game.rs

@@ -389,9 +389,11 @@ mod tests {
         let b = p[1][0];
         let c = p[2][0];
 
-        assert_ulps_eq!(a, Gaussian::from_ms(24.999999, 5.729068), epsilon = 1e-6);
-        assert_ulps_eq!(b, Gaussian::from_ms(25.000000, 5.707423), epsilon = 1e-6);
-        assert_ulps_eq!(c, Gaussian::from_ms(24.999999, 5.729068), epsilon = 1e-6);
+        // Goldens updated for natural-parameter storage: mu rounds to 25.0 (was 24.999999),
+        // sigma shifts by ~3e-7 ULPs (within 1e-6 of original). Both bounded differences.
+        assert_ulps_eq!(a, Gaussian::from_ms(25.0, 5.729069), epsilon = 1e-6);
+        assert_ulps_eq!(b, Gaussian::from_ms(25.0, 5.707424), epsilon = 1e-6);
+        assert_ulps_eq!(c, Gaussian::from_ms(25.0, 5.729069), epsilon = 1e-6);
 
         let t_a = Player::new(
             Gaussian::from_ms(25.0, 3.0),