Guard all _health field accesses with Volatile.Read / Volatile.Write.
ReadAsync, WriteAsync, and ProbeLoopAsync run on different threads and
several updates are read-modify-write (new DriverHealth(_, _health.X, _)).
Without volatile semantics a concurrent update can be lost or a stale
LastSuccessfulRead timestamp propagated. DriverHealth is an immutable
record so Volatile is sufficient — no lock needed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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