Function tier_divergence 

pub fn tier_divergence(
    interp_output: &str,
    interp_error: Option<&str>,
    vm_output: &str,
    vm_error: Option<&str>,
) -> Option<String>

Compare one test computation’s run on the interpreter against its run on the bytecode VM — the --tiers all comparison, PURE so it is unit-lockable with synthetic divergent inputs. Both outputs are newline-joined lines (the shared engine convention), compared byte-for-byte in FULL; error text is compared exactly; the outcome class (clean vs errored) falls out of the error comparison. Returns None on agreement, or the divergence bundle.