metering 0.18.0

German energy metering domain library — DST-correct Europe/Berlin calendar arithmetic incl. the 06:00 Gastag, gas m3→kWh_Hs conversion, SigLinDe gas SLP, billing-period aggregation, Ersatzwertbildung, a robust validation engine, check-digit-validated MaLo-IDs, Bundesland holiday calendar and SLP/RLM/iMSys classification. Zero I/O, no async, no clock, exact decimal quantities.