fix(mtconnect): volatile _options publication, IConvertible write guard, idempotent client dispose

Driver.MTConnect-005/-006/-007, each TDD'd red-first.

- -005: _options was the one field at the StartCoreAsync commit point published by a
  plain write and read lock-free from ReadAsync/WriteAsync/DiscoverAsync/HostName/health.
  Now Volatile.Write/Volatile.Read like _session/_binding/_announcedInstanceId, pinned by
  a source scan (MTConnectOptionsPublicationTests) that fails on any unfenced touch.
- -006: TryFormatValue guarded null and Array, but Convert.ToString renders ANY
  non-IConvertible value as its type name under a Good status. Guard is now the property
  the remark relies on; the accepted set (string/bool/numeric/DateTime) is unchanged.
- -007: MTConnectAgentClient.Dispose was a check-then-set on a non-volatile bool, so two
  racing disposals both passed and the loser cancelled an already-disposed CTS. Now an
  Interlocked.Exchange claim over an int, with the request seams reading it volatile.

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

@@ -370,6 +370,13 @@ internal sealed class CannedAgentClient : IMTConnectAgentClient, IDisposable
     /// <summary>When set, a write throws this instead of answering — the transport-failure leg.</summary>
     public Exception? WriteFailure { get; set; }
 
+    /// <summary>
+    ///     Ran after a write has been recorded and before its outcome is returned. The deterministic
+    ///     injection point for something that must happen BETWEEN two slots of one batch — cancelling
+    ///     the batch's token, for instance — with no timing hope involved.
+    /// </summary>
+    public Action? AfterWrite { get; set; }
+
     /// <inheritdoc/>
     public Task<MTConnectWriteOutcome> WriteObservationAsync(
         string deviceName, string dataItemId, string value, CancellationToken cancellationToken)
@@ -377,6 +384,7 @@ internal sealed class CannedAgentClient : IMTConnectAgentClient, IDisposable
         ObjectDisposedException.ThrowIf(IsDisposed, this);
         cancellationToken.ThrowIfCancellationRequested();
         Writes.Enqueue((deviceName, dataItemId, value));
+        AfterWrite?.Invoke();
 
         return WriteFailure is null ? Task.FromResult(WriteOutcome) : throw WriteFailure;
     }