Phase 7 follow-up #245 — ScriptedAlarmReadable adapter over engine state

Task #245 — exposes each scripted alarm's current ActiveState as IReadable so
OPC UA variable reads on Source=ScriptedAlarm nodes return the live predicate
truth instead of BadNotFound.

## ScriptedAlarmReadable

Wraps ScriptedAlarmEngine + implements IReadable:
- Known alarm + Active → DataValueSnapshot(true, Good)
- Known alarm + Inactive → DataValueSnapshot(false, Good)
- Unknown alarm id → DataValueSnapshot(null, BadNodeIdUnknown) — surfaces
  misconfiguration rather than silently reading false
- Batch reads preserve request order

Phase7EngineComposer.Compose now returns this as ScriptedAlarmReadable when
ScriptedAlarm rows are present. ScriptedAlarmSource (IAlarmSource for the event
stream) stays in place — the IReadable is a separate adapter over the same engine.

## Tests — 6 new + 1 updated composer test = 19 total Phase 7 tests

ScriptedAlarmReadableTests covers: inactive + active predicate → bool snapshot,
unknown alarm id → BadNodeIdUnknown, batch order preservation, null-engine +
null-fullReferences guards. The active-predicate test uses ctx.GetTag on a seeded
upstream value to drive a real cascade through the engine.

Updated Phase7EngineComposerTests to assert ScriptedAlarmReadable is non-null
when alarms compose, null when only virtual tags.

## Follow-ups remaining
- #244 — driver-bridge feed populating CachedTagUpstreamSource
- #246 — Program.cs Compose call + SqliteStoreAndForwardSink lifecycle

tests/ZB.MOM.WW.OtOpcUa.Server.Tests/Phase7/Phase7EngineComposerTests.cs

@@ -72,9 +72,28 @@ public sealed class Phase7EngineComposerTests
             loggerFactory: NullLoggerFactory.Instance);
 
         result.VirtualReadable.ShouldNotBeNull();
+        result.ScriptedAlarmReadable.ShouldBeNull("no alarms configured");
         result.Disposables.Count.ShouldBeGreaterThan(0);
     }
 
+    [Fact]
+    public void Compose_ScriptedAlarm_rows_returns_non_null_ScriptedAlarmReadable()
+    {
+        var scripts = new[] { ScriptRow("scr-1", "return false;") };
+        var alarms = new[] { AlarmRow("al-1", "scr-1") };
+
+        var result = Phase7EngineComposer.Compose(
+            scripts, [], alarms,
+            upstream: new CachedTagUpstreamSource(),
+            alarmStateStore: new InMemoryAlarmStateStore(),
+            historianSink: NullAlarmHistorianSink.Instance,
+            rootScriptLogger: new LoggerConfiguration().CreateLogger(),
+            loggerFactory: NullLoggerFactory.Instance);
+
+        result.ScriptedAlarmReadable.ShouldNotBeNull("task #245 — alarm Active state readable");
+        result.VirtualReadable.ShouldBeNull();
+    }
+
     [Fact]
     public void Compose_missing_script_reference_throws_with_actionable_message()
     {