IMU robustness

[jkaflik]

Context

Current firmware initializes the LSM6DSO once and sets an LED failure flag if initialization fails. Upstream OpenMower LowLevel includes additional initialization checks and retry/failure behavior.

Current Local Behavior

• Supports LSM6DSO on HW 0.13.x.
• Publishes raw accel/gyro as sensor_msgs/Imu at 50 Hz.
• Sets orientation_covariance[0] = -1 because orientation is unavailable.
• Does not retry initialization.

Upstream Reference Behavior

OpenMower LowLevel:

• Supports several IMU variants across board revisions.
• For LSM6DSO, reads WHOAMI and expects 0b01101100.
• Retries initialization multiple times before declaring failure.
• Uses fatal blink/sound behavior when IMU initialization fails.
• Provides an imu_loop() hook for serial/fused IMU variants.

Acceptance Criteria

• Add LSM6DSO WHOAMI validation during init.
• Retry IMU initialization with a bounded retry count and visible status feedback.
• Decide whether IMU init failure should be non-fatal, fatal, or configurable for this ROS 2 firmware.
• Verify accel and gyro axis mapping/signs against upstream OpenMower and ROS frame conventions.
• Document IMU frame id and orientation convention.
• Keep publishing orientation_covariance[0] = -1 while orientation is unavailable.
• Add runtime read failure handling instead of silently skipping forever.

Notes

Comparison found a potential gyro sign difference versus upstream OpenMower LSM6DSO mapping. That should be verified on hardware before changing behavior.