T05: pymodbus client + register map (port HA sigenergy)

Create sigenstor/modbus/ package with: async pymodbus TCP client, register constants ported from Home Assistant sigenergy integration, telemetry read helpers (SoC, battery kW, PV kW, load kW, grid kW, mode, fault), write helpers for EMS mode + charge/discharge power + remote schedule. Serialise writes via asyncio.Lock. Exponential backoff 1/2/4/8s, max 3 retries → event + revert safe self-consume. Depends on T03 settings module (already done).