Since 60d0651a88 every typed request registers a per-connection cancellable
slot that its handler goroutine releases in a deferred cleanup after sending
its result. The server replays a request id when it wants the durable receipt
again, and that replay can reach the reader before the previous handler's
deferred release runs. launchCancellableRequest treated that as a duplicate
and dropped it, so the server waited out the operation's full timeout for a
result the agent already held. The Linux x64 native-verification leg failed
this way on 12 of the last 25 main runs, always on a "replay 1" dispatch of
host update, storage cleanup, or Docker lifecycle.
Give each slot a done channel that closes on release. A replay whose id is
still registered on the same connection now waits for that release and then
runs, answering from the durable receipt. Invalid ids and over-capacity
requests are still dropped. A unit test pins the wait-then-run behaviour and
the agent-lifecycle contract records the replay rule.
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