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/SiteRuntimeOptions.cs

@@ -77,4 +77,17 @@ public class SiteRuntimeOptions
     /// bounded script-execution threads is gone. Default: 30000ms.
     /// </summary>
     public int StuckScriptGraceMs { get; set; } = 30000;
+
+    /// <summary>
+    /// WP2.6d (arch-review misc — site stream alarm-vs-attribute shared buffer):
+    /// capacity of the bounded hand-off queue feeding <see cref="Streaming.SiteStreamManager"/>'s
+    /// dedicated alarm publish path. Alarm state changes are staged through this
+    /// queue into their OWN broadcast source — separate from the (far higher-volume)
+    /// attribute publish source — so an attribute storm cannot evict a pending alarm
+    /// transition. DropOldest overflow; drops are counted (surfaced on the site health
+    /// report) and logged. Default: 2000 (alarm volume is inherently far lower than
+    /// attribute volume, so this rarely needs to be anywhere near
+    /// <see cref="StreamBufferSize"/>).
+    /// </summary>
+    public int AlarmPublishQueueCapacity { get; set; } = 2000;
 }