grid-billing 0.15.0

Role-neutral grid invoice calculation: NNE/KA/MMM/MSB (PIDs 31001/31002/31005/31006/31009/31011) — used by netzbilanzd (NB) and invoicd (LF selbstausstellen). §14a ToU HT/NT. Zero I/O, no float money. Optional `bo4e` feature renders the InvoiceDocument as a rubo4e Rechnung.