Fix EKF IMU input: point imu0 at real MAVROS topic

- ekf.yaml imu0: /imu/data -> /mavros/mavros/data (the empty /imu/data
  topic had no publisher; EKF produced no output)
- Verified: /odometry/filtered now publishes valid fused orientation
  with real Pixhawk IMU via MAVROS
- Note: high covariance expected (single budget IMU, untuned, no position
  input) - tune after Xsens/DVL added

src/rov_navigation/config/ekf.yaml

@@ -12,7 +12,7 @@ ekf_filter_node:
     base_link_frame: base_link
     world_frame: odom
     # Xsens IMU — primary attitude source
-    imu0: /imu/data
+    imu0: /mavros/mavros/data
     imu0_config: [false, false, false,
                   true,  true,  true,
                   false, false, false,