docs: fix stale numerics in t4-margin-factor plan

The plan's prose quoted Z_cav ≈ 0.046827 and log_evidence ≈ -3.0613,
which diverged from the values asserted by the shipped test in
src/factor/mod.rs (-3.062235327364623). Update prose and the matching
code comment to 0.04678 / -3.0622.

docs/superpowers/plans/2026-04-27-t4-margin-factor.md

@@ -49,7 +49,7 @@ A Gaussian `N(m, σ)` constructed via `Gaussian::from_ms(m, σ)`. Multiplication
 **Concrete numerical check for tests:** With cavity `N(0, 6)` and observation `m_obs=5, σ=1`:
 - `D_cav.pi = 1/36 ≈ 0.027778`, `D_cav.tau = 0`.
 - New marginal: `pi = 0.027778 + 1 = 1.027778`, `tau = 0 + 5 = 5`. So `mu = 5 / 1.027778 ≈ 4.864865`, `sigma = 1/sqrt(1.027778) ≈ 0.986394`.
-- `Z_cav = pdf(5, 0, sqrt(36 + 1)) = pdf(5, 0, sqrt(37)) ≈ 0.046827`. So `log_evidence ≈ -3.0613`.
+- `Z_cav = pdf(5, 0, sqrt(36 + 1)) = pdf(5, 0, sqrt(37)) ≈ 0.04678`. So `log_evidence ≈ -3.0622`.
 
 ---
 
@@ -182,7 +182,7 @@ mod tests {
 
         f.propagate(&mut vars);
         let z = f.evidence_cached.unwrap();
-        // pdf(5, 0, sqrt(37)) ≈ 0.046827
+        // pdf(5, 0, sqrt(37)) ≈ 0.04678
         assert!((z - 0.04682752233851171).abs() < 1e-10);
 
         // Subsequent propagations don't change it.