feat(triggers): add WhileTrue fire mode for Conditional/Expression script triggers

Conditional and Expression script triggers gain an optional `mode` field
in their TriggerConfiguration JSON:

- OnTrue (default): unchanged edge/per-change firing. An absent mode field
  parses as OnTrue, so every existing trigger config behaves identically.
- WhileTrue: fires on the false->true edge, then re-fires on a periodic
  timer while the condition holds; stops on the true->false edge. The
  re-fire cadence is the script's MinTimeBetweenRuns; with none configured
  the trigger degrades to a single edge fire and logs a warning.

ScriptActor tracks condition truth state and manages a dedicated
"whiletrue-trigger" timer. ScriptTriggerConfigCodec and ScriptTriggerEditor
round-trip the mode and expose an OnTrue/WhileTrue selector for the two
trigger kinds. Design: docs/plans/2026-05-18-whiletrue-trigger-mode-design.md

Tests: 7 ScriptActor runtime tests (edge fire, timer re-fire, stop,
re-arm, no-MinTimeBetweenRuns degrade, OnTrue regressions) + 14 codec /
editor tests. SiteRuntime suite 206 green, CentralUI suite 295 green.

tests/ScadaLink.CentralUI.Tests/Shared/ScriptTriggerConfigCodecTests.cs

@@ -0,0 +1,137 @@
+using ScadaLink.CentralUI.Components.Shared;
+
+namespace ScadaLink.CentralUI.Tests.Shared;
+
+/// <summary>
+/// Round-trip coverage for the WhileTrue/OnTrue <c>mode</c> field on the
+/// Conditional and Expression script triggers.
+/// </summary>
+public class ScriptTriggerConfigCodecTests
+{
+    // ── Parse: mode field ──────────────────────────────────────────────────
+
+    [Fact]
+    public void Parse_Conditional_WithoutMode_DefaultsToOnTrue()
+    {
+        const string json = @"{""attributeName"":""Temp"",""operator"":"">"",""threshold"":80}";
+
+        var model = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Conditional);
+
+        Assert.Equal(ScriptTriggerMode.OnTrue, model.Mode);
+    }
+
+    [Fact]
+    public void Parse_Conditional_WhileTrue_IsRead()
+    {
+        const string json =
+            @"{""attributeName"":""Temp"",""operator"":"">"",""threshold"":80,""mode"":""WhileTrue""}";
+
+        var model = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Conditional);
+
+        Assert.Equal(ScriptTriggerMode.WhileTrue, model.Mode);
+    }
+
+    [Fact]
+    public void Parse_Expression_WithoutMode_DefaultsToOnTrue()
+    {
+        const string json = @"{""expression"":""Attributes[\""T\""] > 1""}";
+
+        var model = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Expression);
+
+        Assert.Equal(ScriptTriggerMode.OnTrue, model.Mode);
+    }
+
+    [Fact]
+    public void Parse_Expression_WhileTrue_IsRead()
+    {
+        const string json =
+            @"{""expression"":""Attributes[\""T\""] > 1"",""mode"":""WhileTrue""}";
+
+        var model = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Expression);
+
+        Assert.Equal(ScriptTriggerMode.WhileTrue, model.Mode);
+    }
+
+    [Fact]
+    public void Parse_UnrecognizedMode_DefaultsToOnTrue()
+    {
+        const string json =
+            @"{""attributeName"":""Temp"",""operator"":"">"",""threshold"":80,""mode"":""Sometimes""}";
+
+        var model = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Conditional);
+
+        Assert.Equal(ScriptTriggerMode.OnTrue, model.Mode);
+    }
+
+    // ── Serialize: mode field ──────────────────────────────────────────────
+
+    [Fact]
+    public void Serialize_Conditional_WhileTrue_WritesMode()
+    {
+        var model = new ScriptTriggerModel
+        {
+            AttributeName = "Temp",
+            Operator = ">",
+            Threshold = 80,
+            Mode = ScriptTriggerMode.WhileTrue
+        };
+
+        var json = ScriptTriggerConfigCodec.Serialize(model, ScriptTriggerKind.Conditional);
+
+        Assert.Contains("\"mode\":\"WhileTrue\"", json);
+    }
+
+    [Fact]
+    public void Serialize_Expression_WhileTrue_WritesMode()
+    {
+        var model = new ScriptTriggerModel
+        {
+            Expression = "Attributes[\"T\"] > 1",
+            Mode = ScriptTriggerMode.WhileTrue
+        };
+
+        var json = ScriptTriggerConfigCodec.Serialize(model, ScriptTriggerKind.Expression);
+
+        Assert.Contains("\"mode\":\"WhileTrue\"", json);
+    }
+
+    // ── Round-trip ─────────────────────────────────────────────────────────
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void RoundTrip_Conditional_PreservesMode(bool whileTrue)
+    {
+        var mode = whileTrue ? ScriptTriggerMode.WhileTrue : ScriptTriggerMode.OnTrue;
+        var original = new ScriptTriggerModel
+        {
+            AttributeName = "Temp",
+            Operator = ">=",
+            Threshold = 12.5,
+            Mode = mode
+        };
+
+        var json = ScriptTriggerConfigCodec.Serialize(original, ScriptTriggerKind.Conditional);
+        var reparsed = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Conditional);
+
+        Assert.Equal(mode, reparsed.Mode);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void RoundTrip_Expression_PreservesMode(bool whileTrue)
+    {
+        var mode = whileTrue ? ScriptTriggerMode.WhileTrue : ScriptTriggerMode.OnTrue;
+        var original = new ScriptTriggerModel
+        {
+            Expression = "Attributes[\"T\"] > 1",
+            Mode = mode
+        };
+
+        var json = ScriptTriggerConfigCodec.Serialize(original, ScriptTriggerKind.Expression);
+        var reparsed = ScriptTriggerConfigCodec.Parse(json, ScriptTriggerKind.Expression);
+
+        Assert.Equal(mode, reparsed.Mode);
+    }
+}