metering 0.21.0

German energy metering domain library — DST-correct Europe/Berlin calendar arithmetic on both the Liefertag and the 06:00 Gastag, gas m3→kWh_Hs conversion, SigLinDe gas SLP, billing-period aggregation, Ersatzwertbildung, a robust validation engine, para 14a netzorientierte Steuerung and Modul 3 registers, conservation-checked allocation for para 42b/42c, charging sessions onto the settlement grid, check-digit-validated MaLo-IDs and EICs, and SLP/RLM/iMSys classification. Zero I/O, no async, no clock, exact decimal quantities.