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/Server/ZB.MOM.WW.OtOpcUa.AdminUI/ScriptAnalysis/ScriptAnalysisService.cs

@@ -347,7 +347,7 @@ public sealed class ScriptAnalysisService
                     : $"**Tag path** `{Code(info.Path)}`\n\n{info.Kind} · Type **{info.DataType}**"
                         + (info.DriverInstanceId is null ? "" : $" · Driver `{Code(info.DriverInstanceId)}`");
                 // Truthfulness: ctx.SetVirtualTag(...) is a no-op in the live single-tag evaluator
-                // (RoslynVirtualTagEvaluator drops cross-tag writes; the cascade VirtualTagEngine is dormant).
+                // (RoslynVirtualTagEvaluator drops cross-tag writes).
                 if (isSetVirtualTag)
                     tagMd += "\n\n⚠ Cross-tag `SetVirtualTag` writes are currently dropped in single-tag mode " +
                              "(the cascade engine is not wired into the host); this write will not take effect at runtime.";

src/Server/ZB.MOM.WW.OtOpcUa.Host/Engines/RoslynVirtualTagEvaluator.cs

@@ -16,7 +16,7 @@ namespace ZB.MOM.WW.OtOpcUa.Host.Engines;
 ///
 /// Single-tag mode: cross-tag <c>ctx.SetVirtualTag</c> writes are dropped (logged) because
 /// fan-out between actors is owned by <c>DependencyMuxActor</c>, not by the eval engine.
-/// Cycle detection + cascade ordering live in <see cref="VirtualTagEngine"/>; this adapter
+/// Cycle detection lives in the deploy gate (<c>DependencyGraph</c>); this adapter
 /// stays single-tag scoped to keep <see cref="VirtualTagActor"/>'s message loop simple.
 ///
 /// <para>

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs

@@ -366,7 +366,7 @@ public sealed class VirtualTagActor : ReceiveActor
             _virtualTagId, _hasLastValue ? _lastValue : null, timestampUtc, CorrelationId.NewId(), OpcUaStatus.BadInternalError));
     }
 
-    /// <summary>Inputs-ready gate (mirrors the dormant <c>VirtualTagEngine.AreInputsReady</c>): true once
+    /// <summary>Inputs-ready gate (the retired cascade engine's rule): true once
     /// every declared dependency ref has been seen at least once. Called once per failure — the ref list
     /// is small, so no caching. A vacuously-true empty ref list means a no-dependency script degrades on
     /// its first failure.</summary>