Health contract: a write response (per-node refusals included) is the server
answering, so it restores Healthy and advances LastSuccessfulRead; a write with no
reply (transport failure / timeout) writes Degraded with the stamp preserved; a
caller-cancelled write touches nothing. A good keep-alive with no outage or dead
subscription latched now clears a Degraded left by a failed read/write instead of
only moving the stamp (a subscription-only driver otherwise stayed Degraded until
its next re-initialise). All writes are generation-stamped.
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty