pbio/imu: Save one off bias calibration.

When the hub first boots, it is good to have an initial value for the bias to help suppress drift during the first nonstationary seconds.

As an estimate, we can use the bias first saved on the previous boot.

lib/pbio/src/imu.c

@@ -400,6 +400,16 @@ static void pbio_imu_handle_stationary_data_func(const int32_t *gyro_data_sum, c
         // Update bias at decreasing rate.
         gyro_bias.values[i] = gyro_bias.values[i] * (1.0f - weight) + weight * average_now;
     }
+
+    // If persistent gyro bias has never been set, do so now and request saving.
+    // This ensures a better starting point for the next boot. We do this only
+    // once to avoid unnecessary writes on every shutdown. It can be further
+    // refined with a calibration routine performed by the user.
+    if (persistent_settings && !(persistent_settings->flags & PBIO_IMU_SETTINGS_FLAGS_GYRO_BIAS_INITIAL_SET) && stationary_counter > 2) {
+        persistent_settings->angular_velocity_bias_start = gyro_bias;
+        persistent_settings->flags |= PBIO_IMU_SETTINGS_FLAGS_GYRO_BIAS_INITIAL_SET;
+        pbsys_storage_request_write();
+    }
 }
 
 /**