Add switch error rate metric in accuracy tests

[gregorgorjanc]

This is a standard metric - for example, see definition in

Phasing millions of samples achieves near perfect accuracy, enabling parent-of-origin analyses
https://www.cell.com/hgg-advances/fulltext/S2666-2477(25)00082-X

We need to think how to present this as we already have too much information in accuracy tests - see #172