fix(site-runtime): deployment whose Instance Actor dies during init reports Failed, exactly-once reply (S6)

Success now requires BOTH persistence commit AND an InstanceActorInitialized
readiness signal from the actor's PreStart — persistence can commit before the
actor's async init has run or failed, so persistence alone must not report
Success. An actor that dies during init never signals readiness; the Terminated
fallback fails that deployment and rolls back the optimistic state. The
persisted-row rollback is deferred until the store commits so it cannot race the
optimistic write.

Deviation from plan Task 15: the plan's swallow-only guard did not handle the
persistence-first ordering (empirically the store commits before the Terminated
signal, so Success was reported for a dead actor). Added the readiness handshake
to make the join deterministic.

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Messages/InstanceActorInitialized.cs

@@ -0,0 +1,19 @@
+namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Messages;
+
+/// <summary>
+/// Readiness signal an <c>InstanceActor</c> sends to its parent
+/// <c>DeploymentManagerActor</c> at the very end of a successful <c>PreStart</c> —
+/// after its child Script/Alarm/NativeAlarm actors have been created and its DCL
+/// subscriptions issued. If <c>PreStart</c> throws before this point (e.g. a child
+/// actor cannot be created), the message is never sent and the actor is stopped
+/// with an <c>ActorInitializationException</c> instead.
+///
+/// The Deployment Manager uses it to confirm a deployment only once the Instance
+/// Actor has actually initialized — SQLite persistence completing is necessary but
+/// NOT sufficient, because persistence can commit before the actor's asynchronous
+/// init has run (or failed). Reporting deployment <c>Success</c> is therefore gated
+/// on BOTH the persistence result AND this signal (S6); an actor that dies during
+/// init never sends it, so its deployment is failed by the Terminated fallback.
+/// </summary>
+/// <param name="InstanceUniqueName">The unique name of the initialized instance.</param>
+public sealed record InstanceActorInitialized(string InstanceUniqueName);