PR 4.1 — ITagDiscovery via GalaxyRepositoryClient + AlarmRefBuilder

Browse path online. GalaxyDriver now implements ITagDiscovery against the
gateway's GalaxyRepositoryClient (PR 0.1's mxaccessgw browse RPC) and feeds
the address-space builder one folder per gobject + one variable per dynamic
attribute, with alarm-bearing attributes carrying all five sub-attribute refs
the server-level AlarmConditionService (PR 2.2) needs.

Files:
- Browse/IGalaxyHierarchySource.cs — driver-side seam between the discoverer
  and the gateway. Test fakes return canned hierarchies so the discoverer's
  translation logic is exercised without a real gRPC channel.
- Browse/GatewayGalaxyHierarchySource.cs — production wrapper around
  GalaxyRepositoryClient.DiscoverHierarchyAsync (paged internally).
- Browse/GalaxyDiscoverer.cs — translates GalaxyObject → IAddressSpaceBuilder
  calls. Browse name = contained_name (falls back to tag_name); full
  reference = attr.full_tag_reference when set, else tag_name + "." +
  attribute_name. Skips objects/attributes with empty identity.
- Browse/DataTypeMap.cs — mx_data_type → DriverDataType (port from legacy
  GalaxyProxyDriver.MapDataType, same fallback to String for unknown codes).
- Browse/SecurityMap.cs — security_classification → SecurityClassification
  (port from legacy GalaxyProxyDriver.MapSecurity).
- Browse/AlarmRefBuilder.cs — populates the five sub-attribute refs by
  Galaxy convention (.InAlarm/.Priority/.DescAttrName/.Acked/.AckMsg). The
  same convention the legacy GalaxyAlarmTracker hard-coded; concentrated
  here so PR 2.2's service receives complete AlarmConditionInfo rows.

GalaxyDriver:
- Added internal ctor accepting IGalaxyHierarchySource? for test injection.
  Default lazily builds GatewayGalaxyHierarchySource around a
  GalaxyRepositoryClient constructed from options on first DiscoverAsync.
- Owned GalaxyRepositoryClient disposed in Dispose.
- ApiKey resolution is currently a passthrough of ApiKeySecretRef — PR 4.W
  (or follow-up) wires DPAPI-backed secret resolution.

csproj: path-based ProjectReference to mxaccessgw (the user is shipping
that repo on a parallel track; both repos sit side-by-side on the dev box).
Tests project also references MxGateway.Contracts directly to construct
GalaxyObject / GalaxyAttribute fixtures.

Tests: 10 new in Browse/GalaxyDiscovererTests.cs covering folder-per-object,
variable-per-attribute, full-ref defaulting + gw-supplied override, browse-
name fallback, every metadata field propagation, alarm sub-attribute ref
population, non-alarm rows skip MarkAsAlarmCondition, empty-identity skips,
empty-attribute-name skips, end-to-end through GalaxyDriver.DiscoverAsync.
20 total Galaxy tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -15,6 +15,10 @@
     <ItemGroup>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core\ZB.MOM.WW.OtOpcUa.Core.csproj"/>
+        <!-- mxaccessgw .NET client. Path-based ProjectReference because both repos sit
+             side-by-side on the dev box; long-term we'll consume MxGateway.Client as a
+             NuGet package. PR 4.W revisits the dependency shape before parity gating. -->
+        <ProjectReference Include="..\..\..\mxaccessgw\clients\dotnet\MxGateway.Client\MxGateway.Client.csproj"/>
     </ItemGroup>
 
     <ItemGroup>