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.Host/Actors/AkkaHostedService.cs

@@ -836,12 +836,18 @@ akka {{
         // writes (static overrides, native_alarm_state) could be cut off on
         // graceful failover. Names unchanged: deployment-manager-singleton /
         // deployment-manager-proxy.
+        // WP2.6a: the shared cache backing SiteExternalSystemRepository reads
+        // (registered singleton by AddSiteRuntime); DeploymentManagerActor invalidates
+        // it after an artifact deploy applies external-system changes.
+        var externalSystemCache = _serviceProvider
+            .GetService<ZB.MOM.WW.ScadaBridge.SiteRuntime.Repositories.ExternalSystemDefinitionCache>();
         var dm = SingletonRegistrar.Start(
             _actorSystem!, "deployment-manager",
             Props.Create(() => new DeploymentManagerActor(
                 storage, compilationService, sharedScriptLibrary, streamManager,
                 siteRuntimeOptionsValue, dmLogger, dclManager,
-                siteHealthCollector, _serviceProvider, null, deploymentConfigFetcher)),
+                siteHealthCollector, _serviceProvider, null, deploymentConfigFetcher,
+                null, null, externalSystemCache)),
             _logger, role: siteRole);
         var dmProxy = dm.Proxy;