Correct the downlink-write activity contract for compressed frames: a write
that does not complete at once counts, and a frame above the threshold
deflates asynchronously so its completion always counts. The Host compresses
only its own outbound frames; uplink compression remains the peer's choice.
Stabilize the compressed-write liveness test with a 500 ms heartbeat interval
and assert the bounded half: after the downlink stops, missing Pongs still
terminate the socket.
Refs #6476
imccyu
d3a6b8ba6f