Battery status improvements

[jkaflik]

Context

Current firmware publishes sensor_msgs/BatteryState and a couple of auxiliary power topics. Upstream OpenMower LowLevel exposes a richer low-level status packet with explicit status bits and faster status updates.

Current Local Behavior

• Publishes power as sensor_msgs/BatteryState at 1 Hz.
• Publishes power/charge_voltage and power/charger_present at 1 Hz.
• Tracks battery voltage, charge voltage, charge current, battery percent, present flag, power supply status, and health.

Upstream Reference Behavior

OpenMower LowLevel status includes:

• Initialized bit.
• Raspberry Pi power bit.
• Charging enabled bit.
• ESC power bit.
• Rain bit.
• Sound available/busy bits.
• UI board available bit.
• Emergency bitmask.
• Battery voltage, charge voltage, charging current, and battery percentage.
• Status update cycle around 100 ms.

Acceptance Criteria

• Decide on a ROS 2 status representation for low-level board state beyond BatteryState.
• Publish charger voltage, charge current, charger present, charge enabled, and battery percentage consistently.
• Publish board status bits or equivalent structured topics for Pi power, charging enabled, ESC power, rain, UI, and emergency once available.
• Fill more BatteryState fields where meaningful, such as current and design capacity if the sign semantics are clear.
• Review status publish rate. Keep BatteryState at a reasonable rate, but consider faster event/status topics where needed.
• Document topic names and units in README or project documentation.

Notes

Prefer standard ROS messages where they fit, but use explicit custom status only if standard messages become ambiguous.