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/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverTypeNames.cs

@@ -56,6 +56,9 @@ public static class DriverTypeNames
     /// <summary>Read-only SQL Server table/view poller — publishes columns as OPC UA variables.</summary>
     public const string Sql = "Sql";
 
+    /// <summary>MTConnect Agent driver — read-only HTTP/XML over an Agent's probe/current/sample surface.</summary>
+    public const string MTConnect = "MTConnect";
+
     /// <summary>
     ///     Every driver-type string declared above, for callers that need to enumerate
     ///     the full set (e.g. validation of an authored <c>DriverType</c>).
@@ -72,5 +75,6 @@ public static class DriverTypeNames
         Galaxy,
         Calculation,
         Sql,
+        MTConnect,
     ];
 }