Closes the code-review coverage gap: AcknowledgeAsync now has its own
swap-across-the-gate regression test (CallAsync lands on the post-gate
session), the subscribe-path coverage gap is documented, and the bounded
fallback drops from 2s to 250ms (the buggy-path signal is a synchronous
pre-await read, so it always wins well inside the bound).
Joseph Doherty
eede0c926b