fix(optimal_control): type annotation in ou_estimator test

- Add explicit f64 type to dt variable to fix ambiguous sqrt() call
- Kalman filter tests passing successfully

src/optimal_control/ou_estimator.rs

@@ -196,7 +196,7 @@ mod tests {
         let true_kappa = 0.5;
         let true_theta = 0.0;
         let true_sigma = 0.2;
-        let dt = 1.0 / 252.0;
+        let dt: f64 = 1.0 / 252.0;
         let n = 500;
         
         let mut rng = thread_rng();