sidecar: wire IAlarmEventWriter into Program.cs (PR C.2)

Fifth PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Depends on PR C.1
(AahClientManagedAlarmEventWriter), already merged.

Today HistorianFrameHandler is constructed at Program.cs line 57
without an alarmWriter, so every WriteAlarmEvents frame replies
"Sidecar not configured with an alarm-event writer" and the lmxopcua
side keeps the row queued. C.2 wires a real writer behind a new
OTOPCUA_HISTORIAN_ALARM_WRITE_ENABLED toggle.

- Program.BuildAlarmWriter — gated on the env var (default true,
  fail-open under accidental misconfiguration). Constructs an
  AahClientManagedAlarmEventWriter wrapping a
  SdkAlarmHistorianWriteBackend with the same connection config the
  read path uses.
- Install-Services.ps1 — appends OTOPCUA_HISTORIAN_ALARM_WRITE_ENABLED=true
  to the OtOpcUaWonderwareHistorian service env block when the
  sidecar is installed. Read-only deployments flip it to false at
  service-config edit time without re-installing.
- HistorianFrameHandler signature already accepts
  IAlarmEventWriter? — supplying non-null at line 57 lights up
  the WriteAlarmEvents reply path that's been dormant since PR 3.3.

Until PR D.1 pins the live aahClientManaged entry point, the
SdkAlarmHistorianWriteBackend reports RetryPlease for every event
with a structured diagnostic. The lmxopcua-side
SqliteStoreAndForwardSink retains queued events; same effective
behaviour as today's NullAlarmHistorianSink fallback but with
visible diagnostics rather than silent discard.

Tests:
- 6 BuildAlarmWriter env-var cases — unset / true / false /
  unrecognized → default-on / capitalization variants.
- Full sidecar test suite: 56 passed (was 48; 8 new).

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

src/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware/Program.cs

@@ -54,7 +54,8 @@ public static class Program
             }
 
             using var historian = BuildHistorian();
-            var handler = new HistorianFrameHandler(historian, Log.Logger);
+            var alarmWriter = BuildAlarmWriter();
+            var handler = new HistorianFrameHandler(historian, Log.Logger, alarmWriter);
             using var server = new PipeServer(pipeName, allowedSid, sharedSecret, Log.Logger);
 
             Log.Information("Wonderware historian sidecar serving — pipe={Pipe} allowedSid={Sid}", pipeName, allowedSidValue);
@@ -107,4 +108,46 @@ public static class Program
         var raw = Environment.GetEnvironmentVariable(envName);
         return int.TryParse(raw, out var parsed) ? parsed : defaultValue;
     }
+
+    /// <summary>
+    ///     Constructs the alarm-event writer when the alarm-write toggle is on, otherwise
+    ///     returns <c>null</c> so <see cref="HistorianFrameHandler"/> falls back to the
+    ///     "not configured" reply for any incoming <c>WriteAlarmEvents</c> frame.
+    ///     Default is <c>true</c> when <c>OTOPCUA_HISTORIAN_ENABLED=true</c>; explicitly
+    ///     set <c>OTOPCUA_HISTORIAN_ALARM_WRITE_ENABLED=false</c> to keep a read-only
+    ///     deployment that still loads the SDK for reads.
+    /// </summary>
+    internal static IAlarmEventWriter? BuildAlarmWriter()
+    {
+        var raw = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_ALARM_WRITE_ENABLED");
+        var enabled = string.IsNullOrWhiteSpace(raw)
+            ? true
+            : !string.Equals(raw, "false", StringComparison.OrdinalIgnoreCase);
+
+        if (!enabled)
+        {
+            Log.Information("Alarm-event writer disabled (OTOPCUA_HISTORIAN_ALARM_WRITE_ENABLED=false); historian sidecar will reject WriteAlarmEvents frames.");
+            return null;
+        }
+
+        var cfg = BuildAlarmWriterConfig();
+        var backend = new SdkAlarmHistorianWriteBackend(cfg);
+        Log.Information("Alarm-event writer enabled — backend=SdkAlarmHistorianWriteBackend server={Server}", cfg.ServerName);
+        return new AahClientManagedAlarmEventWriter(backend);
+    }
+
+    private static HistorianConfiguration BuildAlarmWriterConfig()
+    {
+        return new HistorianConfiguration
+        {
+            Enabled = true,
+            ServerName = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_SERVER") ?? "localhost",
+            Port = TryParseInt("OTOPCUA_HISTORIAN_PORT", 32568),
+            IntegratedSecurity = !string.Equals(Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_INTEGRATED"), "false", StringComparison.OrdinalIgnoreCase),
+            UserName = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_USER"),
+            Password = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_PASS"),
+            CommandTimeoutSeconds = TryParseInt("OTOPCUA_HISTORIAN_TIMEOUT_SEC", 30),
+            FailureCooldownSeconds = TryParseInt("OTOPCUA_HISTORIAN_COOLDOWN_SEC", 60),
+        };
+    }
 }