feat(siteruntime): event-driven Attributes.WaitAsync attribute-change helper

Adds InstanceActor one-shot waiter registry (fast-path + change-match + scheduled
timeout self-eviction), threads per-script timeout token through ScriptRuntimeContext,
and exposes Attributes.WaitAsync(value|predicate, timeout). Replaces handshake busy-poll.
Implements spec docs/plans/2026-06-17-waitfor-attribute-change-helper-spec.md §3-§5;
§6 routed variant + WaitForAsync + quality-only mode deferred.

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/AlarmExecutionActor.cs

@@ -113,7 +113,12 @@ public class AlarmExecutionActor : ReceiveActor
                     // context's id as its ParentExecutionId — null today, so the
                     // run is a root, but the plumbing exists for a future
                     // firing id.
-                    parentExecutionId: parentExecutionId);
+                    parentExecutionId: parentExecutionId,
+                    // WaitForAttribute (spec §4.4): thread the alarm on-trigger
+                    // script's per-script execution-timeout token so a
+                    // Attributes.WaitAsync inside an on-trigger script is bounded
+                    // by the same script deadline.
+                    scriptTimeoutToken: cts.Token);
 
                 var globals = new ScriptGlobals
                 {