Phase 2 PR 11 — HistoryReadEvents IPC (alarm history). New Shared.Contracts messages HistoryReadEventsRequest/Response + GalaxyHistoricalEvent DTO (MessageKind 0x66/0x67). IGalaxyBackend gains HistoryReadEventsAsync, Stub/DbBacked return canonical pending error, MxAccessGalaxyBackend delegates to _historian.ReadEventsAsync (ported in PR 5) and maps HistorianEventDto → GalaxyHistoricalEvent — Guid.ToString() for EventId wire shape, DateTime → Unix ms for both EventTime (when the event fired in the process) and ReceivedTime (when the Historian persisted it), DisplayText + Severity pass through. SourceName is string? — null means 'all sources' (passed straight through to HistorianDataSource.ReadEventsAsync which adds the AddEventFilter('Source', Equal, ...) only when non-null). Distinct from the live GalaxyAlarmEvent type because historical rows carry both timestamps and lack StateTransition (Historian logs instantaneous events, not the OPC UA Part 9 alarm lifecycle; translating to OPC UA event lifecycle is the alarm-subsystem's job). Guards: null historian → Historian-disabled error; SDK exception → Success=false with message chained. Tests (3 new): disabled-error when historian null, maps HistorianEventDto with full field set (Id/Source/EventTime/ReceivedTime/DisplayText/Severity=900) to GalaxyHistoricalEvent, null SourceName passes through unchanged (verifies the 'all sources' contract). Galaxy.Host.Tests Unit suite 34 pass / 0 fail. Galaxy.Host builds clean. Branches off phase-2-pr10-history-attime since both extend the MessageKind enum; fast-forwards if PR 10 merges first.

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

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

@@ -360,6 +360,46 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend, IDisposable
         }
     }
 
+    public async Task<HistoryReadEventsResponse> HistoryReadEventsAsync(
+        HistoryReadEventsRequest req, CancellationToken ct)
+    {
+        if (_historian is null)
+            return new HistoryReadEventsResponse
+            {
+                Success = false,
+                Error = "Historian disabled — no OTOPCUA_HISTORIAN_ENABLED configuration",
+                Events = Array.Empty<GalaxyHistoricalEvent>(),
+            };
+
+        var start = DateTimeOffset.FromUnixTimeMilliseconds(req.StartUtcUnixMs).UtcDateTime;
+        var end = DateTimeOffset.FromUnixTimeMilliseconds(req.EndUtcUnixMs).UtcDateTime;
+
+        try
+        {
+            var events = await _historian.ReadEventsAsync(req.SourceName, start, end, req.MaxEvents, ct).ConfigureAwait(false);
+            var wire = events.Select(e => new GalaxyHistoricalEvent
+            {
+                EventId = e.Id.ToString(),
+                SourceName = e.Source,
+                EventTimeUtcUnixMs = new DateTimeOffset(DateTime.SpecifyKind(e.EventTime, DateTimeKind.Utc), TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                ReceivedTimeUtcUnixMs = new DateTimeOffset(DateTime.SpecifyKind(e.ReceivedTime, DateTimeKind.Utc), TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                DisplayText = e.DisplayText,
+                Severity = e.Severity,
+            }).ToArray();
+            return new HistoryReadEventsResponse { Success = true, Events = wire };
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            return new HistoryReadEventsResponse
+            {
+                Success = false,
+                Error = $"Historian event read failed: {ex.Message}",
+                Events = Array.Empty<GalaxyHistoricalEvent>(),
+            };
+        }
+    }
+
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse { Accepted = true, GraceSeconds = 15 });