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

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/AlarmSummaryServiceTests.cs

@@ -167,4 +167,40 @@ public class AlarmSummaryServiceTests
         Assert.Equal(0, rollup.WorstSeverity);
         Assert.Equal(0, rollup.UnackedCount);
     }
+
+    // ── BuildFromLiveAlarms (plan #10, Task 5) ────────────────────────────────
+
+    [Fact]
+    public void BuildFromLiveAlarms_FlattensAndSorts_SameAsPollPath_WithEmptyNotReporting()
+    {
+        // Intentionally out-of-order input to prove the deterministic
+        // instance-then-alarm-name sort matches GetSiteAlarmsAsync.
+        var alarms = new List<AlarmStateChanged>
+        {
+            NativeAlarm("zeta", "Zulu", AlarmState.Active, 900, active: true, acked: false),
+            NativeAlarm("alpha", "Bravo", AlarmState.Active, 500, active: true, acked: true),
+            NativeAlarm("alpha", "Alpha", AlarmState.Active, 700, active: true, acked: false),
+        };
+
+        var result = CreateSut().BuildFromLiveAlarms(alarms);
+
+        // One row per alarm, instance taken from the alarm itself.
+        Assert.Equal(3, result.Alarms.Count);
+        Assert.Collection(result.Alarms,
+            r => Assert.Equal(("alpha", "Alpha"), (r.InstanceUniqueName, r.Alarm.AlarmName)),
+            r => Assert.Equal(("alpha", "Bravo"), (r.InstanceUniqueName, r.Alarm.AlarmName)),
+            r => Assert.Equal(("zeta", "Zulu"), (r.InstanceUniqueName, r.Alarm.AlarmName)));
+
+        // The live path can't compute "not reporting" — always empty.
+        Assert.Empty(result.NotReportingInstances);
+    }
+
+    [Fact]
+    public void BuildFromLiveAlarms_EmptySnapshot_YieldsNoRows()
+    {
+        var result = CreateSut().BuildFromLiveAlarms(Array.Empty<AlarmStateChanged>());
+
+        Assert.Empty(result.Alarms);
+        Assert.Empty(result.NotReportingInstances);
+    }
 }