feat(mtconnect): host registration + DriverTypeNames const + guard parity (Task 16)

Register the MTConnect factory + probe with the Host, add the
DriverTypeNames.MTConnect constant, and keep DriverTypeNamesGuardTests green.

- DriverTypeNames: new MTConnect const + appended to All.
- DriverFactoryBootstrap: MTConnectDriverFactoryExtensions.Register in
  Register(...) at the default Tier A (managed HttpClient/XML, in-process;
  Tier C is the only tier arming process-recycle, wrong here), and
  TryAddEnumerable(IDriverProbe -> MTConnectDriverProbe) in
  AddOtOpcUaDriverProbes so the probe reaches admin-only nodes (the
  admin-pinned Test-Connect singleton) without double-registering on a
  fused admin,driver node.
- Host.csproj: ProjectReference to Driver.MTConnect (required to compile
  the Register call).
- Core.Abstractions.Tests.csproj: ProjectReference to Driver.MTConnect so
  the guard's bin scan discovers the factory. This and the constant MUST
  land together -- verified RED (2/4 facts) with the constant alone.
- DriverProbeRegistrationTests: MTConnect added to AdminUiDriverTypeKeys;
  its idempotency fact asserts distinct-probe-count and goes RED without
  this (and RED if TryAddEnumerable is downgraded to AddSingleton).

Reconciles all four "MTConnect" literals onto the one constant: the
factory's DriverTypeName, MTConnectDriver.DriverType, and
MTConnectDriverProbe.DriverType now all alias DriverTypeNames.MTConnect
(no circular reference -- Core.Abstractions is a leaf both driver
projects already reference). This is the repo's TwinCat/Focas anti-drift
convention.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs

@@ -386,7 +386,12 @@ public sealed class MTConnectDriver : IDriver, IReadable, ISubscribable, ITagDis
     public string DriverInstanceId => _driverInstanceId;
 
     /// <inheritdoc/>
-    public string DriverType => "MTConnect";
+    /// <remarks>
+    ///     Sourced from the <see cref="DriverTypeNames"/> constant rather than a literal — the
+    ///     repo-wide fix for the historical <c>TwinCat</c>/<c>Focas</c> drift, where a driver's own
+    ///     spelling diverged from the constant every dispatch map keys off.
+    /// </remarks>
+    public string DriverType => DriverTypeNames.MTConnect;
 
     /// <summary>
     ///     The live <c>dataItemId → snapshot</c> map that backs the <b>subscription</b> surface: