Routine write key capture fetches unordered

A list-returning @routine write (MutationField.MutationRoutineWriteField) produces rows in no defined order, with no directive the author can write to fix it and no diagnostic saying so. Sibling of roadmap/routine-composition-surface-from-facts.md (R704), which fixes the same defect class on the read side; split off because the seam is different (a write’s key capture, not a read’s order surface) and the fix is not the same edit.

The direct-return shape is not exempted from the deterministic-order rule; it is outside the capability the rule keys on. MutationRoutineWriteField implements MutationField alone, not SqlGeneratingField, and GraphitronSchemaValidator.validateListRequiresOrdering guards SqlGeneratingField members only, so the leaf never reaches the check. It also carries no ordering slot, so there is nowhere for a resolved OrderBySpec to live even if @defaultOrder were honoured on the mutation field. Both of its fetches are unordered: step 1’s key capture emits .select(source.<key cols>).from(source).fetch() over the routine result, and step 2 is a keyed SELECT ... WHERE key IN (...) .fetch() that is the field’s visible result.

Mutation.rentFilm in the sakila example schema ([Rental!]! off rent_film) is a live instance.

The carrier shape (Mutation.rentFilmPayload) fails differently and should be checked separately. Its payload data field is an SqlGeneratingField and is exempted, by validateListRequiresOrdering’s `requiresReFetch() clause, whose stated justification is that the ORDER BY idx scatter re-keys the re-projected rows to the upstream source order. That is sound wherever the upstream order is itself defined; for a routine write the upstream is step 1’s unordered fetch, so the exemption rests on a premise this path does not supply.

The two failure modes matter because the fix differs: non-membership is closed by capability membership (or by a membership meta-test that makes such gaps fail loudly), while the exemption is closed by narrowing requiresReFetch().

Open questions for the Spec: