Export shunt/HV monitor telemetry on CAN (0x602–0x605) and implement encoding

[Modellfan]

Motivation

• Provide CAN telemetry for the shunt and HV monitor that mirrors existing serial values and matches the contactor telemetry style (fixed CAN IDs, raw payload bytes, no CRC/counter).
• Close the visibility gap for shunt/HV monitor runtime parameters by publishing scaled/offset uint16 values and meta/DTC fields on CAN.

Description

• Add new CAN message IDs BMS_MSG_SHUNT_HV_META (0x602), BMS_MSG_SHUNT_HV_I_CURRENT_COUNTERS (0x603), BMS_MSG_SHUNT_HV_DI_TEMP_POWER (0x604), and BMS_MSG_SHUNT_HV_U12_U3 (0x605) in settings.h.
• Implement send_shunt_hv_monitor_telemetry_messages() in src/bms/battery_manager.cpp and call it from Monitor100Ms(), producing the four telemetry frames with proper byte-level packing and scaling.
• Add helper encoding utilities encodeUnsigned(), encodeSignedOffset(), and writeUint16ToCanBytes() for consistent scale/offset encoding and saturation to 0..65535 in battery_manager.cpp.
• Include bms/hv_monitor.h and expose the new sender function prototype in battery_manager.h.
• Update documentation files documentation/CAN_Message_Packing.md and documentation/Shunt_HV_Monitor_Telemetry.md with the exact byte layouts and a note that CAN IDs 0x602–0x605 now publish shunt/HV monitor telemetry.

Testing

• No automated tests were added or run for this change.

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Codex Task