Function diff_surfaces 

pub fn diff_surfaces(
    pair: ContextPair,
    a: &IdentitySurface,
    b: &IdentitySurface,
) -> Vec<DriftDiagnostic>

Compare a and b (both Observed) and emit a Vec<DriftDiagnostic> covering every disagreed field. Returns an empty vec when a == b.

pair is recorded into each diagnostic so the report can attribute drift to the right context pair.