Review cleanups on the drain back-off:
- `_drain_after` now requires `guard`: a None default silently meant "release
whatever guard is current", which is the guard-swap bug the parameter fixes.
- The identity rule is stated once (docstring, wrapped), and the redundant
`pending_event is dispatched_event` disjunct is dropped: the same object
always has an equal message_id, and when that id is empty its timestamp
equals itself, so the remaining comparison already covers it.
- Tests drop the `_Adapter` alias and cut the hot-loop matrix from 6 to 4
explicit cases (plain, rewrite with id, rewrite without id, steer). The
demotion route does not interact with the identity axis, and each case
spends a fixed 1s measuring window.
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