fix(virtualtags): a disabled VirtualTag is no longer deployed or evaluated (lmxopcua#787)

VirtualTag.Enabled was authorable but ignored by both runtime-plan seams.
AddressSpaceComposer.Compose and DeploymentArtifact.BuildEquipmentVirtualTagPlans
now drop disabled rows (byte-parity), so no node is materialised and no
VirtualTagActor is spawned - the same outcome as a disabled scripted alarm.
DraftValidator exempts a disabled VT from VirtualTagNoEffectiveTrigger.
Calculation tags have no Enabled flag, so no equivalent gap. docs/VirtualTags.md updated.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Validation/DraftValidator.cs

@@ -493,6 +493,8 @@ public static class DraftValidator
             .ToDictionary(g => g.Key, g => g.First().SourceCode, StringComparer.Ordinal);
         foreach (var v in draft.VirtualTags)
         {
+            // A disabled VirtualTag is never deployed or evaluated (lmxopcua#787), so it can't be stuck waiting.
+            if (!v.Enabled) continue;
             if (v.TimerIntervalMs is > 0) continue;
             var source = scriptSourceById.TryGetValue(v.ScriptId, out var s) ? s : null;
             // An unresolved script is reported elsewhere; with no source there are no reads to count.