H19: Format/InvalidCast/Overflow on a client value never touched the wire but
latched Degraded; a good write set Healthy and so undid a Degraded the poll
loop had just recorded for other tags (the batch read path is worst-wins for
exactly this reason).
- The three conversion failures map to BadTypeMismatch/BadOutOfRange with a
once-per-tag Warning and no health write (AbLegacy's rule).
- A good write (scalar or bit RMW) advances LastSuccessfulRead only - the
device answered - and leaves the state alone.
- A write comms fault (thrown, or a non-refusal status word, which used to
write no health at all) is Degraded with LastSuccessfulRead preserved and
never downgrades Faulted.
Refs #737
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
895cc28886