Implement pymodbus register map for SigenStor (T05)

Complete T05: pymodbus_client.py — TCP Modbus client that reads battery/inverter/PV registers, returns structured dict. Needs connection handling, register definitions for SoC/power/status, error handling, and unit test stub with mocked responses.

Acceptance criteria:

• src/pymodbus_client.py created with ModbusTcpClient class
• Reads battery (SoC), inverter (power, status), PV production registers
• Returns dict from each read cycle
• Handles connection timeout and modbus exceptions
• Unit test stub with mocked responses exists