review(Core.VirtualTags): fix Good-null upstream blocking downstream (Medium)

Re-review at 7286d320. -014 (Medium): AreInputsReady gated on value!=null, so a script
returning null (Good quality) permanently blocked change-triggered dependents at
BadWaitingForInitialData; now gates on the StatusCode Good bit only + test. -015:
TimerTriggerScheduler.Start throws on double-call. -016: fix wrong status-code comment.

src/Core/ZB.MOM.WW.OtOpcUa.Core.VirtualTags/TimerTriggerScheduler.cs

@@ -28,6 +28,7 @@ public sealed class TimerTriggerScheduler : IDisposable
     private readonly CancellationTokenSource _cts = new();
     private long _skippedTickCount;
     private bool _disposed;
+    private bool _started;
 
     /// <summary>Initializes a new instance of the <see cref="TimerTriggerScheduler"/> class.</summary>
     /// <param name="engine">The virtual tag engine to trigger evaluations on.</param>
@@ -52,9 +53,13 @@ public sealed class TimerTriggerScheduler : IDisposable
     ///     behavior.
     /// </summary>
     /// <param name="definitions">The virtual tag definitions to schedule timers for.</param>
+    /// <exception cref="InvalidOperationException">Thrown when <c>Start</c> is called more than once on the same instance.</exception>
     public void Start(IReadOnlyList<VirtualTagDefinition> definitions)
     {
         if (_disposed) throw new ObjectDisposedException(nameof(TimerTriggerScheduler));
+        if (_started) throw new InvalidOperationException(
+            "TimerTriggerScheduler.Start has already been called. Create a new instance for each Load cycle.");
+        _started = true;
 
         var byInterval = definitions
             .Where(d => d.TimerInterval.HasValue && d.TimerInterval.Value > TimeSpan.Zero)

src/Core/ZB.MOM.WW.OtOpcUa.Core.VirtualTags/VirtualTagEngine.cs

@@ -387,17 +387,23 @@ public sealed class VirtualTagEngine : IDisposable
     }
 
     /// <summary>
-    ///     Returns true when every entry in <paramref name="cache"/> has a non-null value
-    ///     and a Good/Uncertain-quality <see cref="DataValueSnapshot.StatusCode"/>. Scripts
-    ///     cast <c>ctx.GetTag(path).Value</c> unconditionally; short-circuiting before the
-    ///     script runs keeps a not-yet-cached upstream from faulting the virtual tag with
-    ///     <c>BadInternalError</c>.
+    ///     Returns true when every entry in <paramref name="cache"/> has a
+    ///     Good/Uncertain-quality <see cref="DataValueSnapshot.StatusCode"/> (bit 31 = 0).
+    ///     Readiness is gated on <c>StatusCode</c> alone — a Good-quality snapshot with a
+    ///     null <c>Value</c> (e.g. a script that returns <c>null</c>, or a
+    ///     <c>ctx.SetVirtualTag</c> call with a null argument) is considered ready and the
+    ///     downstream script is allowed to run. The downstream script will observe the null
+    ///     value and typically throw an NRE, which the outer evaluation catch maps to
+    ///     <c>BadInternalError</c> — the correct sentinel for "script ran but faulted"
+    ///     rather than "inputs not yet available". Previously the null-value check
+    ///     caused a Good-quality null upstream to permanently block all its dependents at
+    ///     <c>BadWaitingForInitialData</c> (Core.VirtualTags-014).
     /// </summary>
     private static bool AreInputsReady(IReadOnlyDictionary<string, DataValueSnapshot> cache)
     {
         foreach (var kv in cache)
         {
-            if (kv.Value is null || kv.Value.Value is null) return false;
+            if (kv.Value is null) return false;
             if ((kv.Value.StatusCode & 0x80000000u) != 0) return false;
         }
         return true;
@@ -470,8 +476,8 @@ public sealed class VirtualTagEngine : IDisposable
         }
         catch
         {
-            // Caller logs + maps to BadTypeMismatch — we let null propagate so the
-            // outer evaluation path sets the Bad quality.
+            // Caller maps null to BadInternalError (0x80020000) — we let null propagate
+            // so the outer evaluation path sets the Bad quality on the snapshot.
             return null;
         }
     }