As a candidate, Zohran Mamdani promised to fund the city’s three library systems with half of 1% of the city’s budget, vowing to end the usual budget dance and financial uncertainty that put libraries at the middle of tense negotiations with the City Council. But as mayor, Mamdani’s executive budget proposal coughs up just .42% of the budget — less than his predecessor, Eric Adams had budgeted — and advocates young and old say the annual budget…