`MapError` — Layer-Boundary Translation (v1.39.0)

Collapse or translate the error union at layer boundaries — e.g. convert a domain union into a single API error type:

// Collapse union into a single domain error for the HTTP layer
Result<Order, DomainError> adapted = result.MapError(e => e.Match(
    v => new DomainError(v.Message),
    i => new DomainError(i.Message),
    p => new DomainError(p.Message),
    d => new DomainError(d.Message)));