sidecar: AahClientManagedAlarmEventWriter implements IAlarmEventWriter (PR C.1)

Fourth PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Independent of Tracks A and B —
the sidecar slot defined in HistorianFrameHandler line 242 is unwired
today; PR C.2 (next) flips it on in Program.cs.

- AlarmHistorianWriteOutcome (sidecar-local, net48 — twin of
  Core.AlarmHistorian.HistorianWriteOutcome which is net10): Ack /
  RetryPlease / PermanentFail.
- IAlarmHistorianWriteBackend abstraction so the SDK call can be
  faked in unit tests.
- AahClientManagedAlarmEventWriter implements IAlarmEventWriter,
  delegates to the backend, maps Ack→true / Retry|Permanent→false
  for the IPC bool[] reply contract. Backend exception → whole
  batch RetryPlease (preserves the sender's queue across transients
  rather than dropping). Wrong-count return defends against a
  backend bug desyncing queue accounting.
- SdkAlarmHistorianWriteBackend — production binding skeleton.
  Reports RetryPlease for every event and logs a structured
  diagnostic until PR D.1 pins the live aahClientManaged entry
  point against the dev rig. The sender's SqliteStoreAndForwardSink
  retains queued events, mirroring today's NullAlarmHistorianSink
  behaviour but with visible diagnostics instead of silent discard.
- MapOutcome shared helper — pinned via theory tests so the D.1
  swap can change the SDK call site without reshuffling the
  HRESULT → outcome mapping.

Tests:
- 6 writer tests — empty batch / single Ack / mixed Ack-Retry-
  Permanent-Ack ordering / backend-throw → RetryPlease batch /
  cancellation propagates / wrong-count defensive degrade.
- 5 outcome theory cases — hresult 0 → Ack, malformed wins over
  hresult 0, comm error → Retry, unknown failure → Retry,
  malformed + comm → Permanent.
- Full sidecar test suite: 48 passed (was 42; 6 new).

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

src/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware/Backend/IAlarmHistorianWriteBackend.cs

@@ -0,0 +1,30 @@
+using System.Threading;
+using System.Threading.Tasks;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Ipc;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Backend
+{
+    /// <summary>
+    ///     The actual aahClientManaged-bound writer. Extracted so unit tests can
+    ///     substitute a fake without touching the SDK; the production
+    ///     implementation lives in <see cref="SdkAlarmHistorianWriteBackend"/>.
+    /// </summary>
+    /// <remarks>
+    ///     Implementations are responsible for connection management + cluster
+    ///     failover. The wrapping <see cref="AahClientManagedAlarmEventWriter"/>
+    ///     handles batch-level orchestration but delegates the per-event SDK call
+    ///     here so the unit tests can drive every documented MxStatus outcome
+    ///     without an installed AVEVA Historian.
+    /// </remarks>
+    public interface IAlarmHistorianWriteBackend
+    {
+        /// <summary>
+        ///     Persist the supplied events to the historian. Returns one outcome per
+        ///     input slot in the same order — must always return an array of the same
+        ///     length as <paramref name="events"/>.
+        /// </summary>
+        Task<AlarmHistorianWriteOutcome[]> WriteBatchAsync(
+            AlarmHistorianEventDto[] events,
+            CancellationToken cancellationToken);
+    }
+}