fix(mtconnect): read-path restart publishes the new cursor; document the review trade-offs (review m6, m1, m4, m5, m8)

- m6: a /current read that sees a new Agent instanceId now also publishes the new
  process's cursor (generation-checked, captured with the read's session), so a
  later subscription does not open at the dead instance's sequence and flicker
  through a clear + re-baseline. Pinned in the T-2 test.
- Documented (code + docs/drivers/MTConnect.md): m1 the bounded stale-read window of
  a stream stalled without failing; m4 a refused multi-pair PUT's retry may re-send
  an already-applied pair (harmless); m5 a non-Agent 200 now reads reachable to the
  ping; m8 a refused raised count takes one transient blip before the step-back.
- m10 confirmed: no host key is persisted (DriverHostStatus dropped, #521) - the
  redacted HostName only changes an in-memory /hosts key for a query-bearing URI.

Refs #743

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectReadPathTests.cs

@@ -130,6 +130,10 @@ public sealed class MTConnectReadPathTests
 
             raised.ShouldBe(1, "once per distinct instanceId, not once per read");
             driver.CachedProbeModel.ShouldBeNull("the model describes a process that no longer exists");
+            // Review m6: the session's cursor moves to the new process, so a later subscription does not open at the
+            // dead instance's sequence (and flicker through a clear + re-baseline on its first chunk).
+            driver.AgentInstanceId.ShouldBe(RestartedInstance);
+            driver.AgentNextSequence.ShouldBe(5);
         }
         finally
         {