Implement pymodbus register map for Sigenergy SigenStor battery system

Create pymodbus TCP client in sigentrol that reads all relevant battery, inverter, and PV registers. Based on SigenStor 16-24kW Modbus register map. Target file: src/pymodbus_client.py

Acceptance criteria:

• pymodbus_client.py implements ModbusTcpClient with connect/disconnect
• Register read methods for battery SoC, power, inverter status, PV production
• Returns structured data (dict) from each read cycle
• Error handling for connection timeout and modbus exceptions
• Unit test stub with mocked responses

Project: sigenstor-control Tags: [sigenstor, phase1, pymodbus]