* Keep connections alive through brief daemon slowdowns
The active connection's liveness check was a side effect of the
candidate-probe loop: a single 5s latency measurement doubled as the
death timeout, so a daemon that was alive but briefly stalled (e.g. a
busy event loop) was misread as dead and the connection was torn down.
Give DaemonClient its own self-scheduling heartbeat that solely owns
liveness (15s timeout, 2 consecutive misses), and make the probe
read-only on the active connection — it reads the heartbeat's last RTT
instead of pinging. Split the shared ping into a pure measurement path
(measureLatency) and a private liveness path, so nothing outside the
client can drive teardown.
* Don't let a candidate latency timeout count as a liveness failure
The heartbeat (livenessPing) and candidate ranking (measureLatency) share
one in-flight ping slot. If a heartbeat tick lands while a candidate
measurement is still in flight, the heartbeat dedupes onto it — and a
measurement timeout was then recorded as a liveness failure, nudging the
connection one step closer to a spurious teardown.
Tag each ping with whether its timeout may drive a liveness failure
(only the heartbeat sets it) and gate recordLivenessFailure on that flag,
so an adopted measurement probe can never contribute to teardown.
