Function check_deterministic_composition 

pub fn check_deterministic_composition(journal: &Journal) -> CheckResult

deterministic-composition — prefix stability (docs/context-reuse.md §1), finally checkable: two prompts rendering the identical frame set (same identities, same representations, same order) must compose to the identical composition_digest. A harness whose composition wobbles under an unchanged set is silently destroying the prompt-cache economics the canonical order exists to buy.

Skipped when the journal records no composition digests — the field is optional precisely so a recorder can adopt the vocabulary before wiring composed-prefix hashing.