fix(sensors): disable SENS_MAG_AUTOCAL by default, keep for CAN nodes (#27659)

PR #18421 added the initial mag calibration save and was deliberately merged with SENS_MAG_AUTOCAL=0 until parameter system concerns were addressed. PR #19818 (CAN GPS nodes) flipped the global default to 1 as an undiscussed side change, shipped in v1.14.0. Since then every uncalibrated mag marks itself calibrated (CAL_MAGn_ID set) shortly after the disarmed bias estimator stabilizes, so ground stations stop prompting for compass calibration on new setups, and the first flight happens on a hard-iron-only estimate with an assumed mount rotation.

Restore the disabled default and enable it where it is genuinely needed: CAN nodes, which have no GCS calibration flow and no EKF for in-flight refinement. MBE_ENABLE is untouched, the bias estimator still runs and corrects everywhere.

Signed-off-by: Jacob Dahl <dahl.jakejacob@gmail.com>

ROMFS/cannode/init.d/rcS

@@ -107,6 +107,8 @@ then
 	# conservative mag bias estimation
 	param set-default MBE_LEARN_GAIN 5
 	param set-default IMU_GYRO_CUTOFF 20
+	# nodes have no GCS calibration flow, persist the estimated bias
+	param set-default SENS_MAG_AUTOCAL 1
 	mag_bias_estimator start
 fi
 

src/modules/sensors/sensor_params_mag.yaml

@@ -78,7 +78,9 @@ parameters:
       description:
         short: Magnetometer auto calibration
         long: Automatically initialize magnetometer calibration from bias estimate
-          if available.
+          if available. The estimate only captures hard-iron offsets, so a full
+          calibration is still recommended. Mainly intended for remote nodes
+          (e.g. CAN GPS units) that cannot be calibrated from a ground station.
       category: System
       type: boolean
-      default: 1
+      default: 0