OpenMower ROS firmware heartbeats

[jkaflik]

Context

Upstream OpenMower LowLevel relies on a heartbeat from the high-level stack. If no heartbeat is received within 500 ms, firmware marks ROS as not running and latches emergency. This firmware has micro-ROS agent reconnect handling, but no high-level heartbeat/failsafe contract yet.

Upstream Reference Behavior

OpenMower LowLevel:

• Receives ll_heartbeat packets from the high-level stack.
• Uses a 500 ms heartbeat timeout.
• Latches emergency when heartbeat is missing.
• Allows high-level code to request emergency latch.
• Allows high-level code to request emergency release.
• Tracks ROS-running state for LEDs, UI, sound, and safety behavior.

Acceptance Criteria

• Define ROS 2 API for firmware heartbeat from OpenMower ROS.
• Track last heartbeat timestamp independently from micro-ROS agent connection state.
• Latch emergency when heartbeat timeout expires.
• Publish firmware/high-level connection state.
• Support high-level emergency request.
• Support high-level emergency release request, coordinated with physical emergency inputs.
• Make timeout configurable or clearly document the default.
• Add LED/status indication for heartbeat timeout.
• Document the heartbeat contract so OpenMower ROS can implement the matching publisher/client.

Notes

This should complement micro-ROS agent reconnect. Agent connected does not necessarily mean the mower high-level controller is alive and safe.