Fix mypy: add explicit ndarray type annotations for class attributes

mypy infers different ndarray shape types for np.zeros(0) vs np.array([...]),
causing type mismatch errors in subclasses. Explicit np.ndarray annotations
make the types compatible.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: adeebshihadeh

rednose/helpers/kalmanfilter.py

@@ -5,9 +5,9 @@
 
 class KalmanFilter:
   name = "<name>"
-  initial_x = np.zeros(0)
-  initial_P_diag = np.zeros(0)
-  Q = np.zeros((0, 0))
+  initial_x: np.ndarray = np.zeros(0)
+  initial_P_diag: np.ndarray = np.zeros(0)
+  Q: np.ndarray = np.zeros((0, 0))
   obs_noise: dict[int, Any] = {}
 
   # Should be initialized when initializating a KalmanFilter implementation