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.

docs/requirements/Component-TemplateEngine.md

@@ -55,8 +55,8 @@ Central cluster only. Sites receive flattened output and have no awareness of te
 
 ### Script (Template-Level)
 - Name, Lock Flag, C# source code.
-- Trigger configuration: Interval, Value Change, Conditional, or invoked by alarm/other script.
-- Optional minimum time between runs.
+- Trigger configuration: Interval, Value Change, Conditional, Expression, or invoked by alarm/other script. Conditional and Expression triggers also carry a fire mode — **OnTrue** (fire as the condition becomes true) or **WhileTrue** (re-fire on a timer while it stays true).
+- Optional minimum time between runs — also the re-fire cadence for a WhileTrue trigger.
 - **Parameter Definition** *(optional)*: Defines input parameters (name and data type per parameter). Scripts without parameters accept no arguments.
 - **Return Value Definition** *(optional)*: Defines the structure of the script's return value (field names and data types). Supports single objects and lists of objects. Scripts without a return definition return void.