The FIFO-conservation invariant fired on the loop-authored continuation
reason path: a continue-with-reason decision entered the steering FIFO
without an agent/inbox/enqueue, so its later dequeue/discard had no
matching enqueue. Emit the enqueue for that steer too, add a regression
test that mounts the invariant over a continue-with-reason turn and a
cancel, and hoist the duplicated inboxInfo helper into inbox.ts.
Found by fresh-eye review.