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/ScriptExecutionActor.cs

@@ -221,7 +221,12 @@ public class ScriptExecutionActor : ReceiveActor
                     // M2.12 (#25): thread the singleton site event logger so
                     // recursion-limit violations at CallScript/CallShared emit a
                     // script Error site event in addition to ILogger.LogError.
-                    siteEventLogger: siteEventLogger);
+                    siteEventLogger: siteEventLogger,
+                    // WaitForAttribute (spec §4.3/§4.4): thread the per-script
+                    // execution-timeout token so Attributes.WaitAsync's Ask is
+                    // bounded by the script's own ExecutionTimeoutSeconds — a
+                    // shorter script deadline wins over the wait's own timeout.
+                    scriptTimeoutToken: cts.Token);
 
                 var globals = new ScriptGlobals
                 {