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.SiteRuntime/Actors/DeploymentManagerActor.cs

@@ -16,6 +16,7 @@ using ZB.MOM.WW.ScadaBridge.SiteEventLogging;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Deployment;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Messages;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Persistence;
+using ZB.MOM.WW.ScadaBridge.SiteRuntime.Repositories;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Streaming;
 
@@ -40,6 +41,14 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
     // DeployInstanceCommand is validated before any actor/persistence work.
     private readonly DeployCompileValidator _deployCompileValidator;
     private readonly SharedScriptLibrary _sharedScriptLibrary;
+    /// <summary>
+    /// WP2.6a: the shared cache backing <c>SiteExternalSystemRepository</c> reads.
+    /// Invalidated wholesale after <see cref="HandleDeployArtifacts"/> applies
+    /// external-system changes so a script's next call re-reads fresh definitions/method
+    /// lists instead of serving a stale cached snapshot. Null in tests that do not wire
+    /// external-system caching — the invalidation call is then simply skipped.
+    /// </summary>
+    private readonly ExternalSystemDefinitionCache? _externalSystemCache;
     private readonly SiteStreamManager? _streamManager;
     private readonly SiteRuntimeOptions _options;
     private readonly ILogger<DeploymentManagerActor> _logger;
@@ -157,6 +166,12 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
     /// deployed configuration at startup; defaults to 5 seconds when null.</param>
     /// <param name="configLoader">Optional override for loading all deployed configurations
     /// at startup; defaults to reading from <paramref name="storage"/>. Primarily for tests.</param>
+    /// <param name="externalSystemCache">
+    /// WP2.6a: the shared <see cref="ExternalSystemDefinitionCache"/> backing
+    /// <c>SiteExternalSystemRepository</c> reads, invalidated after this actor applies
+    /// external-system changes. Optional/null in tests that do not exercise external-system
+    /// caching.
+    /// </param>
     public DeploymentManagerActor(
         SiteStorageService storage,
         ScriptCompilationService compilationService,
@@ -170,12 +185,14 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
         ILoggerFactory? loggerFactory = null,
         IDeploymentConfigFetcher? configFetcher = null,
         TimeSpan? startupLoadRetryInterval = null,
-        Func<Task<List<DeployedInstance>>>? configLoader = null)
+        Func<Task<List<DeployedInstance>>>? configLoader = null,
+        ExternalSystemDefinitionCache? externalSystemCache = null)
     {
         _storage = storage;
         _compilationService = compilationService;
         _deployCompileValidator = new DeployCompileValidator(compilationService);
         _sharedScriptLibrary = sharedScriptLibrary;
+        _externalSystemCache = externalSystemCache;
         _streamManager = streamManager;
         _options = options;
         _dclManager = dclManager;
@@ -1914,6 +1931,11 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
 
                     await _storage.DeleteExternalSystemsExceptAsync(
                         command.ExternalSystems.Select(es => es.Name).ToList());
+
+                    // WP2.6a: drop the shared read cache so the next script call sees
+                    // the freshly-applied definitions/method lists instead of a stale
+                    // snapshot loaded before this deploy.
+                    _externalSystemCache?.InvalidateAll();
                 }
 
                 // Store database connection definitions