fix(virtualtags): consume VirtualTag.TimerIntervalMs; add VirtualTagNoEffectiveTrigger gate (lmxopcua#783)

Choice: implement the timer (option a). The field was designed and promised as a
working trigger: the entity doc says ChangeTriggered and TimerIntervalMs together
realize the trigger model, the DB check constraint requires ChangeTriggered OR a
timer >= 50, the AdminUI virtual-tag modal exposes "TimerIntervalMs (optional)" and
UnsTreeService rejects a tag with neither, and the 2026-06-08 design specified
"ChangeTriggered OR TimerIntervalMs>=50". Only the runtime never honoured it (the
standalone TimerTriggerScheduler was deleted in #603), so a timer-only tag sat at
BadWaitingForInitialData forever.

- EquipmentVirtualTagPlan carries ChangeTriggered/TimerIntervalMs (in equality, so
  an edit respawns the child); both the composer and the artifact decode fill them.
- VirtualTagActor schedules a repeating tick (PreStart, cancelled in PostStop,
  clamped to the 50 ms floor). ChangeTriggered=false makes the timer the only
  trigger. A tick during an in-flight async run (#779) is dropped, not stashed, so
  slow scripts never build a backlog of runs.
- DraftValidator VirtualTagNoEffectiveTrigger (hard error, same severity as
  CalculationNoEffectiveTrigger): no timer and no change trigger with a literal
  ctx.GetTag read.

Tests: VirtualTagActorTimerTests, DraftValidatorVirtualTagTriggerTests, artifact
decode test. Mutation-checked: no scheduling, stashing ticks, no cancel in
PostStop, and removing the validator call each fail a test.

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

tests/Core/ZB.MOM.WW.OtOpcUa.Configuration.Tests/DraftValidatorVirtualTagTriggerTests.cs

@@ -0,0 +1,57 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
+using ZB.MOM.WW.OtOpcUa.Configuration.Validation;
+
+namespace ZB.MOM.WW.OtOpcUa.Configuration.Tests;
+
+/// <summary>lmxopcua#783 — <c>VirtualTagNoEffectiveTrigger</c>, the VirtualTag mirror of
+/// <c>CalculationNoEffectiveTrigger</c>.</summary>
+[Trait("Category", "Unit")]
+public sealed class DraftValidatorVirtualTagTriggerTests
+{
+    private static Script ScriptRow(string id, string source) => new()
+    {
+        ScriptId = id, Name = id, SourceCode = source, SourceHash = "h-" + id,
+    };
+
+    private static VirtualTag Vt(bool changeTriggered, int? timer, string scriptId = "s1") => new()
+    {
+        VirtualTagId = "vt-1", EquipmentId = "eq-1", Name = "V", DataType = "Double", ScriptId = scriptId,
+        ChangeTriggered = changeTriggered, TimerIntervalMs = timer,
+    };
+
+    private static IEnumerable<ValidationError> Run(VirtualTag vt, string source) =>
+        DraftValidator.Validate(new DraftSnapshot
+        {
+            GenerationId = 1, ClusterId = "c",
+            VirtualTags = [vt],
+            Scripts = [ScriptRow("s1", source)],
+        }).Where(e => e.Code == "VirtualTagNoEffectiveTrigger");
+
+    [Fact]
+    public void Change_triggered_with_no_read_and_no_timer_is_rejected() =>
+        Run(Vt(true, null), "return 1;").ShouldHaveSingleItem().Context.ShouldBe("vt-1");
+
+    [Fact]
+    public void Not_change_triggered_and_no_timer_is_rejected() =>
+        Run(Vt(false, null), "return (double)ctx.GetTag(\"a/b\").Value;").ShouldHaveSingleItem();
+
+    [Fact]
+    public void A_timer_alone_is_an_effective_trigger() =>
+        Run(Vt(true, 1000), "return 1;").ShouldBeEmpty();
+
+    [Fact]
+    public void A_literal_read_with_change_trigger_is_an_effective_trigger() =>
+        Run(Vt(true, null), "return (double)ctx.GetTag(\"a/b\").Value;").ShouldBeEmpty();
+
+    [Fact]
+    public void An_unresolved_script_reports_no_trigger_error()
+    {
+        var errors = DraftValidator.Validate(new DraftSnapshot
+        {
+            GenerationId = 1, ClusterId = "c", VirtualTags = [Vt(true, null, "s-missing")],
+        });
+        errors.ShouldNotContain(e => e.Code == "VirtualTagNoEffectiveTrigger");
+    }
+}