feat(site-runtime): InstanceActor trace arm stamps the device echo of a traced write; Trace rides on published changes

Flow Debugger (design 2026-09-05 §5.3): a traced SetStaticAttributeCommand on a data-sourced
attribute records (attribute → writer, expiry) for TraceArmWindowSeconds (new option,
default 30, validated > 0); the next DCL echo for that attribute is published with the
writer's SourceExecutionId + Trace whatever its value, and the arm is consumed once. Static
writes publish Trace directly. Untraced writes never arm (pinned).

Claude-Session: https://claude.ai/code/session_01GSLkkHKz9JrAtmJjmQK3cP

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptions.cs

@@ -232,4 +232,15 @@ public class SiteRuntimeOptions
     /// (the row-67 <c>EnsureInstanceStartedCommand</c> path is unaffected). Default: 60.
     /// </summary>
     public int InstanceSweepIntervalSeconds { get; set; } = 60;
+
+    /// <summary>
+    /// Flow Debugger trace arm window (design 2026-09-05 §5.3). When a TRACED script write
+    /// targets a data-sourced attribute, the Instance Actor remembers
+    /// <c>(attribute → writer's ExecutionId)</c> for this many seconds; the next DCL value
+    /// change for that attribute — whatever the value — is published stamped with that
+    /// <c>SourceExecutionId</c> and <c>Trace</c>, and the arm is consumed. An arm that expires
+    /// unconsumed emits nothing. Only traced writes arm, so production behaviour is unchanged.
+    /// Default: 30.
+    /// </summary>
+    public int TraceArmWindowSeconds { get; set; } = 30;
 }