feat(devx): check:engine-double-contract gains the consumer-seam refusal invariant (#8194) (#8423)

* feat(devx): check:engine-double-contract gains the consumer-seam existence invariant (#8194)

* test(devx): self-test coverage for the consumer-seam invariant (#8194)

* test(devx): pin scalarWhereIdOf directly — the end-to-end $in assertion was passing for the wrong reason (#8194)

* test(devx): cover the two other mint spellings — the new Error fixture left the call arm untested (#8194)

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Co-authored-by: Claude <noreply@anthropic.com>

Author: Jack Q