refactor(galaxy): migrate to ZB.MOM.WW.MxGateway.* nupkg packages

Imports the freshly-rebuilt ZB.MOM.WW.MxGateway.Client + ZB.MOM.WW.MxGateway.Contracts
nupkgs (0.1.0) from /tmp/mxgw-dist. Replaces the vendored libs/ DLLs and the
pre-restructure MxGateway.* namespaces across the runtime Galaxy driver,
Galaxy.Browser, and their tests.

Key changes:
- nuget-packages/ added as a local feed via NuGet.config; .gitignore exempts it
  from the *.nupkg rule so the packages are tracked
- Directory.Packages.props pins both packages at 0.1.0
- 4 csprojs swap <Reference HintPath="libs/...dll"/> for <PackageReference/>
- 36 .cs files renamed `using MxGateway.*` -> `using ZB.MOM.WW.MxGateway.*`
- libs/ removed (vendored DLLs + README.md)

GalaxyBrowseSession rewritten around the new lazy API:
- RootAsync calls GalaxyRepositoryClient.BrowseAsync (returns LazyBrowseNodes)
  and caches them by TagName instead of bulk-fetching the whole hierarchy
- ExpandAsync looks up the cached LazyBrowseNode and calls its ExpandAsync,
  giving true one-wire-call-per-click instead of in-memory parent/child scan
- _byGobjectId + _hasChildrenSet dropped (LazyBrowseNode carries HasChildrenHint)
- AttributesAsync unchanged (already uses DiscoverHierarchyAsync MaxDepth=0)

Tests: Galaxy.Tests 245/245, Galaxy.Browser.Tests 10/10, AdminUI.Tests 66/66.
Pre-existing 12 solution errors unchanged (test sinks + Cli XML comments).

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browser/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browser.csproj

@@ -13,23 +13,8 @@
         <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browser.Tests" />
     </ItemGroup>
     <ItemGroup>
-        <!-- Vendored mxaccessgw .NET client — same DLLs as Driver.Galaxy.
-             See Driver.Galaxy/libs/README.md for the unwinding plan. -->
-        <Reference Include="MxGateway.Client">
-            <HintPath>..\ZB.MOM.WW.OtOpcUa.Driver.Galaxy\libs\MxGateway.Client.dll</HintPath>
-            <Private>true</Private>
-        </Reference>
-        <Reference Include="MxGateway.Contracts">
-            <HintPath>..\ZB.MOM.WW.OtOpcUa.Driver.Galaxy\libs\MxGateway.Contracts.dll</HintPath>
-            <Private>true</Private>
-        </Reference>
-    </ItemGroup>
-    <ItemGroup>
-        <!-- Transitive deps of the vendored MxGateway.Client.dll -->
-        <PackageReference Include="Google.Protobuf" />
-        <PackageReference Include="Grpc.Core.Api" />
-        <PackageReference Include="Grpc.Net.Client" />
+        <PackageReference Include="ZB.MOM.WW.MxGateway.Client" />
+        <PackageReference Include="ZB.MOM.WW.MxGateway.Contracts" />
         <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
-        <PackageReference Include="Polly.Core" />
     </ItemGroup>
 </Project>