$χ$-independence for K3-surfaces via $p$-adic integration

This article provides a proof of a previously unknown case of Toda's $\chi$-independence conjecture by reduction to non-archimedean local fields. Our strategy is based on a novel comparison of Frobenius-traces for BPS sheaves on moduli spaces of objects in 2-Calabi-Yau categories and the integral of the complex-exponentiated Hasse invariant of the obstruction gerbe. This result applies to many cases of interest, including Nakajima quiver varieties, moduli of Higgs bundles and moduli of sheaves on K3 surfaces. Along the way, we describe the local structure of these moduli stacks and spaces over a base of large mixed characteristic.