PR 4.2 — IReadable abstraction + StatusCodeMap + MxValueDecoder

Read path scaffold + the byte→uint quality mapping table that the parity
matrix (PR 5.x) pins. PR 4.4 supplies the production GW-backed reader; this
PR ships the abstraction and the supporting infrastructure so 4.4 just
plugs the implementation in.

Files:
- Runtime/StatusCodeMap.cs — explicit OPC DA quality byte → OPC UA
  StatusCode uint mapping. Extends the legacy Galaxy.Host
  HistorianQualityMapper with named constants (Good / GoodLocalOverride,
  Uncertain + 4 substatuses, Bad + 7 substatuses, BadInternalError) and an
  MxStatusProxy → uint helper that honors success flag → detail byte →
  detected_by transport-error fallback. Unknown bytes fall back to category
  bucket with a once-per-session diagnostic log so field captures can
  extend the table.
- Runtime/MxValueDecoder.cs — gateway MxValue → boxed CLR value for the
  seven Galaxy data types (Boolean, Int32, Int64, Float32, Float64, String,
  DateTime) plus their array variants. Honors MxValue.IsNull and
  RawValue passthrough.
- Runtime/IGalaxyDataReader.cs — driver-side seam for one-shot reads. PR
  4.4 ships the production wrapper around MxGatewaySession.SubscribeBulk +
  StreamEvents + UnsubscribeBulk; this PR exposes the contract so
  GalaxyDriver.ReadAsync wires through it.
- Runtime/GalaxyMxSession.cs — wrapper around MxGatewaySession that owns
  the Register handle. ConnectAsync opens session + Register; AttachForTests
  lets tests bypass real gw construction. PR 4.3/4.4/4.5 add write,
  subscribe, and reconnect surfaces.

GalaxyDriver:
- Implements IReadable. ReadAsync routes through the injected
  IGalaxyDataReader (test seam) when present; production path throws
  NotSupportedException pointing at PR 4.4 — protects deployments running
  this PR from silent wrong reads while signaling that the legacy-host
  backend (Galaxy:Backend=legacy-host) handles reads in the meantime.
- Internal ctor extended with optional dataReader parameter (default null,
  preserves PR 4.0/4.1 callers).

Tests: 42 new — exhaustive byte→uint table for StatusCodeMap (15 known
codes + category-bucket fallback for unknowns + MxStatusProxy precedence
rules + OPC UA top-byte invariants), every MxValue oneof case for the
decoder (bool/int32/int64/float/double/string/timestamp/3 array variants/
raw bytes/null), GalaxyDriver IReadable wiring (route-through, empty-
request, no-reader-throws, post-dispose-throws, status-code preservation).
62 Galaxy tests total pass.

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

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -4,6 +4,7 @@ using MxGateway.Client;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
 
@@ -22,7 +23,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
 ///     <see cref="GalaxyDriverFactoryExtensions"/> registers under driver-type name
 ///     "GalaxyMxGateway" so both paths can be live simultaneously during parity testing.
 /// </remarks>
-public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
+public sealed class GalaxyDriver : IDriver, ITagDiscovery, IReadable, IDisposable
 {
     private readonly string _driverInstanceId;
     private readonly GalaxyDriverOptions _options;
@@ -35,6 +36,13 @@ public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
     private IGalaxyHierarchySource? _hierarchySource;
     private GalaxyRepositoryClient? _ownedRepositoryClient;
 
+    // PR 4.2 — IGalaxyDataReader is the test seam for IReadable. PR 4.4 supplies the
+    // production implementation that wraps GalaxyMxSession's SubscribeBulk + StreamEvents
+    // pump; until then ReadAsync throws NotSupportedException when the reader is null
+    // (legacy-host backend handles reads in production via DriverNodeManager's
+    // capability-routing).
+    private readonly IGalaxyDataReader? _dataReader;
+
     private DriverHealth _health = new(DriverState.Unknown, null, null);
     private bool _disposed;
 
@@ -42,19 +50,20 @@ public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
         string driverInstanceId,
         GalaxyDriverOptions options,
         ILogger<GalaxyDriver>? logger = null)
-        : this(driverInstanceId, options, hierarchySource: null, logger)
+        : this(driverInstanceId, options, hierarchySource: null, dataReader: null, logger)
     {
     }
 
     /// <summary>
-    ///     Test-visible ctor — inject a custom <see cref="IGalaxyHierarchySource"/> so
-    ///     <see cref="DiscoverAsync"/> can be exercised against canned hierarchies without
-    ///     building a real gRPC channel.
+    ///     Test-visible ctor — inject custom seams so <see cref="DiscoverAsync"/> +
+    ///     <see cref="ReadAsync"/> can be exercised against canned data without building
+    ///     real gRPC channels.
     /// </summary>
     internal GalaxyDriver(
         string driverInstanceId,
         GalaxyDriverOptions options,
         IGalaxyHierarchySource? hierarchySource,
+        IGalaxyDataReader? dataReader = null,
         ILogger<GalaxyDriver>? logger = null)
     {
         _driverInstanceId = !string.IsNullOrWhiteSpace(driverInstanceId)
@@ -63,6 +72,7 @@ public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
         _options = options ?? throw new ArgumentNullException(nameof(options));
         _logger = logger ?? NullLogger<GalaxyDriver>.Instance;
         _hierarchySource = hierarchySource;
+        _dataReader = dataReader;
     }
 
     /// <inheritdoc />
@@ -130,6 +140,31 @@ public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
         await discoverer.DiscoverAsync(builder, cancellationToken).ConfigureAwait(false);
     }
 
+    // ===== IReadable (PR 4.2 — abstraction; PR 4.4 supplies production reader) =====
+
+    /// <inheritdoc />
+    public Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(_disposed, this);
+        ArgumentNullException.ThrowIfNull(fullReferences);
+        if (fullReferences.Count == 0) return Task.FromResult<IReadOnlyList<DataValueSnapshot>>([]);
+
+        if (_dataReader is null)
+        {
+            // The production GW-backed reader builds on the StreamEvents pump that PR 4.4
+            // ships; until then a real gateway-driver instance can't fulfill reads.
+            // Tests that need to exercise IReadable inject a fake reader via the internal
+            // ctor; production deployments running on this PR should keep the
+            // legacy-host backend selected via the Galaxy:Backend flag (PR 4.W).
+            throw new NotSupportedException(
+                "GalaxyDriver.ReadAsync requires the StreamEvents-backed reader from PR 4.4. " +
+                "Until that lands, route reads through the legacy-host backend (Galaxy:Backend=legacy-host).");
+        }
+
+        return _dataReader.ReadAsync(fullReferences, cancellationToken);
+    }
+
     /// <summary>
     ///     Lazily builds the default <see cref="IGalaxyHierarchySource"/> from
     ///     <c>_options.Gateway</c>. Owned <see cref="GalaxyRepositoryClient"/> is disposed in