Undo a declaration: put the field back to the value it had before this app wrote it.

The declaration lapses now instead of at its expires_at, and the reconciler writes back the value it read from the platform before this app's write. will_restore says whether such a value was captured: a declaration that was never applied has nothing to undo and closes without a write. The declaration's status reads state reverted once the restore is written, and last_error says if the platform refused it. A declaration mid-write answers 409 in_flight; retry shortly. If someone else has changed the field since it was applied and the merchant has not settled it, the answer is 409 overwritten_externally and nothing is reverted: release instead to leave their change. Only the declaring app can revert.