At the start of his term in January, Mayor Mamdani had the Big Apple’s lenders in his corner — despite his plans to remake the Big Apple economy under his socialist agenda. Now that appears to be changing fast, as municipal bond investors have begun to sell New York City debt, with prices falling and interest rates — the so-called yields — spiking to their highest levels in months. To be sure, lots of factors are at play here, not least the Iran war that’s hitting bonds across the board. But