No read or write loop checked _plcDead between items: on a frozen-but-
established peer the first op blew its deadline and marked the handle dead,
and every remaining span / tag / write still went to the wire and could wait
its own deadline under _gate (a 20-span pass ~100 s), with Degraded and the
comms-loss watchdog's clock deferred until the pass ended.
- The span loop, the multi-var chunk loop, the per-tag loop and the write
loop stamp BadCommunicationError without I/O once _plcDead is set.
- A multi-var chunk that dies on a transport loss (socket / IO / timeout /
ConnectionError) marks the handle dead instead of falling back to one
doomed per-tag read per member. The framing class is deliberately excluded
there: snap7/S7.Net report a multi-var with one refused item as
ReadData(WrongNumberReceivedBytes) over a healthy stream.
- Live gate via an in-test freezable TCP proxy (counts request frames, not
seconds). Mutation shows S7.Net 0.20 already self-closes on a cancelled op,
so the gate pins the end-to-end property; the driver guard itself is proven
by the unit tests (an op that ignores its token keeps the socket open).
Refs #742
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty