perf(s7): fail the rest of a pass fast once the handle is dead (R07:P1)

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7ArrayWriteAndHealthTests.cs

@@ -243,7 +243,9 @@ public sealed class S7ArrayWriteAndHealthTests
         var (drv, factory) = await TwoTagDriver();
         using var _ = drv;
         factory.Created[0].WriteFaults["DB1.DBW0"] = new PlcException(ErrorCode.WriteData, "Accessing object not allowed");
-        factory.Created[0].WriteFaults["DB1.DBW2"] = new IOException("socket glitch");
+        // A Degraded failure that is NOT connection-fatal: since R07:P1 a fatal one (an IOException)
+        // would fail the rest of the batch without I/O, and C must reach the wire to be the success.
+        factory.Created[0].WriteFaults["DB1.DBW2"] = new PlcException(ErrorCode.SendData, "device glitch");
 
         var res = await drv.WriteAsync(
             [new WriteRequest("A", (ushort)1), new WriteRequest("B", (ushort)2), new WriteRequest("C", (ushort)3)],