perf(misc): cached hot-path lookups, bounded observer queue, alarm-priority stream path

WP2.6 (arch-review remediation, cross-cutting misc):
- SiteExternalSystemRepository: name/ID-indexed ExternalSystemDefinitionCache replaces
  the fetch-all + reverse-map scan on every by-ID/method lookup; loaded once per
  redeploy, invalidated by DeploymentManagerActor after HandleDeployArtifacts applies
  external-system changes. Static JsonSerializerOptions for method-list parsing.
- Inbound API: short-TTL ApiMethodCache fronts the per-request ApiMethod repository
  fetch; invalidated by name via the existing ScriptArtifactChangeSubscriber/
  IScriptArtifactChangeBus pipeline, self-healing via TTL for changes the bus
  doesn't cover (e.g. Management API edits).
- StoreAndForward: the cached-call audit-observer queue — the one unbounded channel
  left in the system — is now bounded (ObserverQueueCapacity, default 10,000) with
  DropOldest overflow and a dropped-notification counter.
- SiteStreamManager: alarm state changes now travel a dedicated publish
  source/broadcast hub, isolated from the (far higher-volume) attribute path, so an
  attribute storm can no longer evict a pending alarm transition; the alarm hand-off
  queue is bounded with a drop counter surfaced on the site health report
  (SiteStreamAlarmDropCount via the new SiteStreamAlarmDropReporter), and publishing
  is skipped entirely at zero subscribers on either path.
- CLI ManagementHttpClient: explicit 30s HttpClient.Timeout on the shared
  construction (was the 100s framework default), overridable via
  SCADABRIDGE_HTTP_TIMEOUT_SECONDS.

Deviation: the failback-probe heartbeat item is NOT included — its only viable
surface (CentralChannelProvider.cs / heartbeat consumers) lives entirely in the
Communication project, explicitly off-limits to this work package this phase.

Tests: SiteRuntime.Tests (550), InboundAPI.Tests (278), StoreAndForward.Tests (133),
CLI.Tests (390), HealthMonitoring.Tests (97) — all green after full solution build.

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/ISiteHealthCollector.cs

@@ -178,6 +178,22 @@ public interface ISiteHealthCollector
         // Default no-op so test fakes do not need to be updated.
     }
 
+    /// <summary>
+    /// WP2.6d (arch-review misc — site stream alarm-vs-attribute shared buffer):
+    /// replace the latest cumulative count of alarm state changes dropped from
+    /// <c>SiteStreamManager</c>'s dedicated alarm publish queue, used by the next
+    /// <see cref="CollectReport"/> call. Refreshed periodically by the site-side
+    /// <c>SiteStreamAlarmDropReporter</c> hosted service. Point-in-time: NOT reset on
+    /// <see cref="CollectReport"/>. Default interface implementation is a no-op so
+    /// existing test fakes continue to compile without per-fake updates.
+    /// </summary>
+    /// <param name="count">The cumulative dropped-alarm count from <c>SiteStreamManager.AlarmPublishDroppedCount</c>.</param>
+    void SetSiteStreamAlarmDropCount(long count)
+    {
+        // Default no-op so test fakes do not need to be updated. The real
+        // SiteHealthCollector overrides this with the Interlocked.Exchange store.
+    }
+
     /// <summary>
     /// Sets the hostname of this node.
     /// </summary>

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/SiteHealthCollector.cs

@@ -39,6 +39,10 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _scriptQueueDepth;
     private int _scriptBusyThreads;
     private long _scriptOldestBusyAgeBits = BitConverter.DoubleToInt64Bits(double.NaN);
+    // WP2.6d: cumulative alarm-publish-queue drop count, refreshed by
+    // SiteStreamAlarmDropReporter. Point-in-time (not reset on collect), like the
+    // scheduler gauges above.
+    private long _siteStreamAlarmDropCount;
     private volatile string _nodeHostname = "";
     private volatile IReadOnlyList<Commons.Messages.Health.NodeStatus>? _clusterNodes;
     private volatile bool _isActiveNode;
@@ -173,6 +177,12 @@ public class SiteHealthCollector : ISiteHealthCollector
         return double.IsNaN(value) ? null : value;
     }
 
+    /// <inheritdoc />
+    public void SetSiteStreamAlarmDropCount(long count)
+    {
+        Interlocked.Exchange(ref _siteStreamAlarmDropCount, count);
+    }
+
     /// <inheritdoc />
     public void SetNodeHostname(string hostname) => _nodeHostname = hostname;
 
@@ -277,7 +287,8 @@ public class SiteHealthCollector : ISiteHealthCollector
             // not run) leaves both report fields null — "no data", not "disconnected
             // with an empty backlog".
             LocalDbReplicationConnected = localDbReplication?.Item1,
-            LocalDbOplogBacklog = localDbReplication?.Item2
+            LocalDbOplogBacklog = localDbReplication?.Item2,
+            SiteStreamAlarmDropCount = Interlocked.Read(ref _siteStreamAlarmDropCount)
         };
     }
 }