Phase 2 PR 10 — HistoryReadAtTime IPC surface. New Shared.Contracts messages HistoryReadAtTimeRequest/Response (MessageKind 0x64/0x65), IGalaxyBackend gains HistoryReadAtTimeAsync, Stub/DbBacked return canonical pending error, MxAccessGalaxyBackend delegates to _historian.ReadAtTimeAsync (ported in PR 5, exposed now) — request timestamp array is flow-encoded as Unix ms to avoid MessagePack DateTime quirks then re-hydrated to DateTime on the Host side. Per-sample mapping uses the same ToWire(HistorianSample) helper as ReadRawAsync so the category→StatusCode mapping stays consistent (Quality byte 192+ → Good 0u, 64-191 → Uncertain, 0-63 → Bad 0x80000000u). Guards: null historian → "Historian disabled" (symmetric with other history paths); empty timestamp array short-circuits to Success=true, Values=[] without an SDK round-trip; SDK exception → Success=false with the message chained. Proxy-side IHistoryProvider.ReadAtTimeAsync capability doesn't exist in Core.Abstractions yet (OPC UA HistoryReadAtTime service is supported but the current IHistoryProvider only has ReadRawAsync + ReadProcessedAsync) — this PR adds the Host-side surface so a future Core.Abstractions extension can wire it through without needing another IPC change. Tests (4 new): disabled-error when historian null, empty-timestamp short-circuit without SDK call, Unix-ms↔DateTime round-trip with Good samples at two distinct timestamps, missing sample (Quality=0) maps to 0x80000000u Bad category. Galaxy.Host.Tests Unit suite: 31 pass / 0 fail (4 new at-time + 27 pre-existing). Galaxy.Host builds clean. Branches off v2.

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

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/DbBackedGalaxyBackend.cs

@@ -136,6 +136,15 @@ public sealed class DbBackedGalaxyBackend(GalaxyRepository repository) : IGalaxy
             Values = System.Array.Empty<GalaxyDataValue>(),
         });
 
+    public Task<HistoryReadAtTimeResponse> HistoryReadAtTimeAsync(
+        HistoryReadAtTimeRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadAtTimeResponse
+        {
+            Success = false,
+            Error = "MXAccess + Historian code lift pending (Phase 2 Task B.1)",
+            Values = System.Array.Empty<GalaxyDataValue>(),
+        });
+
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse { Accepted = true, GraceSeconds = 15 });