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.Commons/Messages/Health/SiteHealthReport.cs

@@ -104,6 +104,17 @@ public record SiteHealthReport(
     /// pair as perfectly healthy. Collapsing null to 0 anywhere on this path is a bug.
     /// </remarks>
     public long? LocalDbOplogBacklog { get; init; }
+
+    /// <summary>
+    /// WP2.6d (arch-review misc — site stream alarm-vs-attribute shared buffer):
+    /// cumulative count of alarm state changes dropped from
+    /// <c>SiteStreamManager</c>'s dedicated alarm publish queue because it was at
+    /// capacity — i.e. the site-wide alarm hand-off itself fell behind, not a
+    /// per-subscriber buffer. Point-in-time (not reset on collect), refreshed by
+    /// the site-side <c>SiteStreamAlarmDropReporter</c> hosted service. Defaults to
+    /// 0 so existing producers/tests that don't populate it stay valid.
+    /// </summary>
+    public long SiteStreamAlarmDropCount { get; init; }
 }
 
 /// <summary>