Mayor Zohran Mamdani’s pied-à-terre tax — his early effort to deliver a core campaign promise — is poised to take effect next month. But the devil is in the details. Implementing the surcharge on second homes with market values higher than $5 million won’t be an easy task, as the city’s Department of Finance and the homeowners subject to the tax are likely to clash over a range of issues related to residency status, valuation and rule compliance.