Function tier_differential 

pub fn tier_differential(source: &str) -> TierVerdict

Run source on every always-available execution tier — the tree-walker oracle and the register VM — and classify the result (Wave 13; the engine behind largo test --tiers all and the F18 “works-on-my-tier” guard).

Output is compared modulo the one benign, documented tier convention (the VM terminates output with a newline the tree-walker omits — trim_end); error text is compared exactly. For sequential programs, as the differential harness is; a concurrency program (which the plain VM entry declines) is out of scope and simply reports Agree from the oracle.