Cover board support

[jkaflik]

Context

The OpenMower LowLevel firmware supports the stock/custom CoverUI board over a second serial link. This firmware currently defines PIN_UI_TX and PIN_UI_RX, but does not implement Cover board communication.

Upstream Reference Behavior

OpenMower LowLevel CoverUI protocol supports:

• Version request/response.
• Button events.
• Emergency state events from stock CoverUI.
• Rain value events from stock CoverUI.
• Topic subscriptions from CoverUI.
• LED matrix updates.
• CRC validation and COBS framing through PacketSerial.

Acceptance Criteria

• Initialize the UI serial port on PIN_UI_TX and PIN_UI_RX for HW 0.13.x.
• Implement packet framing and CRC validation compatible with OpenMower CoverUI protocol.
• Detect UI board availability using version request/response.
• Publish CoverUI button events to ROS 2.
• Accept CoverUI emergency states and feed them into firmware emergency handling.
• Accept CoverUI rain values and feed them into rain detection.
• Send useful LED status updates to the CoverUI board.
• Expose UI board availability in board status telemetry.

Notes

This is separate from onboard NeoPixel LED status. The protocol structs in upstream ui_board.h are a useful compatibility reference.