ParseResult:
    | { ok: true; value: CauslModel }
    | { ok: false; path: readonly (string | number)[]; reason: string }

Result of parseCauslModel — discriminated by ok.

Path-precision is the contract the validator commits to: when a document fails, the caller receives the JSON path to the offending field, not a generic "invalid IR". This is the failure shape SPEC §16.2 names: a wire format with no version discipline is a bug-shaped contract; structured rejection on mismatch is what makes the discipline mechanical.