feat(sitestream): add additive SubscribeSite alarm-only gRPC stream + server handler (plan #10 T2)

Adds an additive, site-wide, alarm-only gRPC stream backing the aggregated
Alarm Summary. Proto: new SiteStreamRequest { correlation_id } message + rpc
SubscribeSite(SiteStreamRequest) returns (stream SiteStreamEvent) on
SiteStreamService — purely additive, no field renumbering. Regenerated the
checked-in SiteStreamGrpc/*.cs.

Server: SubscribeInstance and the new SubscribeSite now delegate to a shared
RunSubscriptionStreamAsync helper (readiness/shutdown guards, correlation-id
validation, duplicate replacement, concurrency cap, bounded DropOldest channel,
relay actor, SiteConnectionOpened/Closed telemetry, guaranteed cleanup). The
only variation is the subscribe delegate: SubscribeSite calls
ISiteStreamSubscriber.SubscribeSiteAlarms (no per-instance filter). Added
SubscribeSiteAlarms to the ISiteStreamSubscriber contract (SiteStreamManager
already implements it from T1). StreamRelayActor reused unchanged — it already
drops IsConfiguredPlaceholder rows and maps the enriched AlarmStateUpdate.

Tests: SubscribeSite subscribes site alarms + removes on cancel, rejects unsafe
correlation ids, and relays a domain AlarmStateChanged as a proto
AlarmStateUpdate on the stream.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/ISiteStreamSubscriber.cs

@@ -17,6 +17,16 @@ public interface ISiteStreamSubscriber
     /// <returns>A subscription ID that can be used for unsubscription.</returns>
     string Subscribe(string instanceName, IActorRef subscriber);
 
+    /// <summary>
+    /// Subscribes an actor to receive ALARM events for ALL instances on the site
+    /// (no per-instance filter). Only <c>AlarmStateChanged</c> events are
+    /// forwarded; attribute-value events are dropped. Backs the site-wide
+    /// <c>SubscribeSite</c> gRPC stream used by the aggregated Alarm Summary.
+    /// </summary>
+    /// <param name="subscriber">The actor reference that will receive alarm stream event messages.</param>
+    /// <returns>A subscription ID that can be used for unsubscription.</returns>
+    string SubscribeSiteAlarms(IActorRef subscriber);
+
     /// <summary>
     /// Removes all subscriptions for the given actor.
     /// </summary>