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

src/ZB.MOM.WW.OtOpcUa.Server/Phase7/ScriptedAlarmReadable.cs

@@ -0,0 +1,58 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.ScriptedAlarms;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Phase7;
+
+/// <summary>
+///     <see cref="IReadable"/> adapter exposing each scripted alarm's current
+///     <see cref="AlarmActiveState"/> as an OPC UA boolean. Phase 7 follow-up (task #245).
+/// </summary>
+/// <remarks>
+///     <para>
+///         Paired with the <see cref="NodeSourceKind.ScriptedAlarm"/> dispatch in
+///         <c>DriverNodeManager.OnReadValue</c>. Full-reference lookup is the
+///         <c>ScriptedAlarmId</c> the walker wrote into <c>DriverAttributeInfo.FullName</c>
+///         when emitting the alarm variable node.
+///     </para>
+///     <para>
+///         Unknown alarm ids return <c>BadNodeIdUnknown</c> so misconfiguration surfaces
+///         instead of silently reading <c>false</c>. Alarms whose predicate has never
+///         been evaluated (brand new, before the engine's first cascade tick) report
+///         <see cref="AlarmActiveState.Inactive"/> via <see cref="AlarmConditionState.Fresh"/>,
+///         which matches the Part 9 initial-state semantics.
+///     </para>
+/// </remarks>
+public sealed class ScriptedAlarmReadable : IReadable
+{
+    /// <summary>OPC UA <c>StatusCodes.BadNodeIdUnknown</c> — kept local so we don't pull the OPC stack.</summary>
+    private const uint BadNodeIdUnknown = 0x80340000;
+
+    private readonly ScriptedAlarmEngine _engine;
+
+    public ScriptedAlarmReadable(ScriptedAlarmEngine engine)
+    {
+        _engine = engine ?? throw new ArgumentNullException(nameof(engine));
+    }
+
+    public Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(fullReferences);
+
+        var now = DateTime.UtcNow;
+        var results = new DataValueSnapshot[fullReferences.Count];
+        for (var i = 0; i < fullReferences.Count; i++)
+        {
+            var alarmId = fullReferences[i];
+            var state = _engine.GetState(alarmId);
+            if (state is null)
+            {
+                results[i] = new DataValueSnapshot(null, BadNodeIdUnknown, null, now);
+                continue;
+            }
+            var active = state.Active == AlarmActiveState.Active;
+            results[i] = new DataValueSnapshot(active, 0u, now, now);
+        }
+        return Task.FromResult<IReadOnlyList<DataValueSnapshot>>(results);
+    }
+}