feat(alarm-summary): live-cache-driven updates with poll fallback (plan #10 T5)

Wire the operator Alarm Summary page to the transient per-site live alarm
cache (ISiteAlarmLiveCache, T4). Live-cache-first: on site select the page
subscribes and rebuilds rows/rollup from near-real-time onChanged deltas; the
15s poll is kept untouched as the authority for NotReporting and as the
safety net whenever the cache is not live (pre-seed or degraded stream). Both
paths mutate shared state only via the Blazor dispatcher, so they never race,
and each rebuild is an idempotent snapshot.

- IAlarmSummaryService.BuildFromLiveAlarms: flattens a live AlarmStateChanged
  snapshot to AlarmSummaryRows with the same deterministic instance-then-name
  sort as GetSiteAlarmsAsync; NotReporting always empty on the live path.
- AlarmSummary.razor: inject ISiteAlarmLiveCache; subscribe on select,
  re-subscribe on site change, unsubscribe on leave + Dispose.
- Tests: service BuildFromLiveAlarms flatten/sort/empty; page subscribe,
  poll-fallback render, onChanged rebuild, unsubscribe on leave/change/dispose.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/IAlarmSummaryService.cs

@@ -42,6 +42,28 @@ public interface IAlarmSummaryService
     /// </returns>
     Task<AlarmSummaryResult> GetSiteAlarmsAsync(int siteId, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Builds an <see cref="AlarmSummaryResult"/> directly from an in-memory live
+    /// snapshot (plan #10, Task 5) — the current alarms served by
+    /// <see cref="ZB.MOM.WW.ScadaBridge.Communication.ISiteAlarmLiveCache.GetCurrentAlarms"/>
+    /// — instead of fanning out per-instance snapshot Asks. Each
+    /// <see cref="AlarmStateChanged"/> is flattened to one
+    /// <see cref="AlarmSummaryRow"/> (instance taken from
+    /// <see cref="AlarmStateChanged.InstanceUniqueName"/>) and ordered with the SAME
+    /// deterministic instance-then-alarm-name sort as
+    /// <see cref="GetSiteAlarmsAsync"/>, so the two paths are interchangeable.
+    /// </summary>
+    /// <remarks>
+    /// The live cache carries only alarm state, so it cannot know which Enabled
+    /// instances are silent-but-reporting versus not reporting at all;
+    /// <see cref="AlarmSummaryResult.NotReportingInstances"/> is therefore always
+    /// empty on this path. The page keeps its periodic <see cref="GetSiteAlarmsAsync"/>
+    /// poll as the authority for "not reporting" (and as a live-stream safety net).
+    /// </remarks>
+    /// <param name="alarms">The current live alarm snapshot for a site.</param>
+    /// <returns>An <see cref="AlarmSummaryResult"/> whose <see cref="AlarmSummaryResult.NotReportingInstances"/> is empty.</returns>
+    AlarmSummaryResult BuildFromLiveAlarms(IReadOnlyList<AlarmStateChanged> alarms);
+
     /// <summary>
     /// Pure roll-up over a set of <see cref="AlarmSummaryRow"/>s. Exposed so the
     /// page (and tests) can recompute the headline tiles without re-querying.