refactor(virtualtags): retire dormant VirtualTagEngine/Source/Definition + TimerTriggerScheduler (#603)

TimerTriggerScheduler was welded to the engine and has no production consumer.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverAttributeInfo.cs

@@ -39,7 +39,7 @@ namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 /// </param>
 /// <param name="VirtualTagId">
 ///     Set when <paramref name="Source"/> is <see cref="NodeSourceKind.Virtual"/> — stable
-///     logical id the VirtualTagEngine addresses by. Null otherwise.
+///     logical id the virtual-tag runtime addresses by. Null otherwise.
 /// </param>
 /// <param name="ScriptedAlarmId">
 ///     Set when <paramref name="Source"/> is <see cref="NodeSourceKind.ScriptedAlarm"/> —
@@ -78,7 +78,7 @@ public sealed record DriverAttributeInfo(
 ///     Discriminates which runtime subsystem owns this node's Read/Write/
 ///     Subscribe dispatch. <c>Driver</c> = a real IDriver capability surface;
 ///     <c>Virtual</c> = a Phase 7 <see cref="DriverAttributeInfo"/>.VirtualTagId'd tag
-///     computed by the VirtualTagEngine; <c>ScriptedAlarm</c> = a scripted Part 9 alarm
+///     computed by the virtual-tag runtime; <c>ScriptedAlarm</c> = a scripted Part 9 alarm
 ///     materialized by the ScriptedAlarmEngine.
 /// </summary>
 public enum NodeSourceKind