fix(purchase): fail a plan step's root row atomically with its reason and suppression release #1052

failExistingPlanStep (internal/purchase/notifications.go) fails a plan step's root execution row in two steps: the pinned compare-and-set to failed (TransitionExecutionStatusPinned, which has no transaction variant) commits first, then a separate transaction stamps Error and NoSubmission and deletes the row's suppressions.

If the process dies between the two, the row stays failed without the reason or the no-submission proof, and its suppressions stay until they expire. adoptableRootRow only adopts pending or notified roots, so no later tick repairs it.

It fails closed: suppressions only hide recommendations, and a failed row without NoSubmission is treated as outcome unknown. Nothing is bought.

The missing reason predates #905. The finding-2 fix on #905 adds the suppression release, so a crash can now also leave those suppressions in place on rows that carry recommendations.

Fix direction: add a transaction variant of TransitionExecutionStatusPinned so the CAS, the stamp and the suppression release commit together.

Tracked in known_issues/36_plan_step_fail_cas_not_atomic.md. Refs #631, #905.