fix(alarms): atomic snapshot+truncation read; direct tests for the flag plumbing (review)

Review found AlarmDispatcher.SnapshotActiveAlarms reading the snapshot and the
truncation verdict through two independent lock acquisitions, defended by a
comment claiming read-order made a race "widen only, never narrow". That claim
was false: a not-truncated -> truncated poll landing between the two reads pairs
a stale false with a capped snapshot, which is exactly the false all-clear the
feature exists to prevent. It was safe only because AlarmCommandHandler
STA-serializes consumer calls — an accident of the call graph, not an invariant.

Made the invariant structural instead of documented. IMxAccessAlarmConsumer now
exposes ONE accessor, `IReadOnlyList<MxAlarmSnapshotRecord> SnapshotActiveAlarms(
out bool truncated)`, which implementations must satisfy from a single
acquisition of the lock guarding the retained snapshot — mirroring the write
side, where FoldFetch already updates snapshot and verdict together. The
separate LastSnapshotTruncated property is gone from every layer, so there is no
second read left to pair badly. `out` over a result struct follows the file's
established idiom (FoldFetch, ParseSnapshotXml).

The same threading applies one level up: IAlarmCommandHandler.QueryActive now
carries `out bool snapshotTruncated`, so MxAccessCommandExecutor stamps the reply
payload from the value the records were stamped with rather than reading the
state a second time.

Direct tests for the three hops that were only covered end-to-end:
- AlarmDispatcherTests: truncated consumer snapshot stamps FromTruncatedSnapshot
  on every mapped record, with a complete-snapshot control, plus an assertion
  that the independent per-record Degraded flag is not dragged along.
- AlarmCommandHandlerTests: the verdict delegates through the dispatcher
  (Theory over both values), and survives a prefix filter that removes every
  record — the case the per-record flag cannot cover.
- AlarmCommandExecutorTests: the reply payload's SnapshotTruncated comes from the
  handler (Theory over both values), including the zero-record case.
The WnWrapAlarmConsumer truncation tests now assert through
SnapshotActiveAlarms(out ...) rather than an internal field, because the pairing
is the contract.

Also: GatewayAlarmMonitor's _snapshotTruncated comment now says "as of the last
full reconcile" rather than implying it tracks the current _alarms contents,
which live transitions keep moving via ApplyTransition between passes.

Detection heuristic still untouched (fetchedRecordCount >= maxAlarmsPerFetch);
no @COUNT parsing, per docs/AlarmProbeFindings.md. Still additive gateway
metadata about our fetch mechanics, not MXAccess behavior — not a parity
deviation, and no event is synthesized.

Gateway: NonWindows.slnx builds clean (0 warnings); ~Alarm filter 107/107 pass.
Worker + Worker.Tests are windev-gated; the signature change was reviewed by
inspection across all 7 IMxAccessAlarmConsumer implementers, all 3
IAlarmCommandHandler implementers, and every call site.

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmDispatcher.cs

@@ -154,15 +154,24 @@ public sealed class AlarmDispatcher : IDisposable
     ///     <see cref="ActiveAlarmSnapshot"/> protos for the
     ///     <c>QueryActiveAlarms</c> RPC's ConditionRefresh stream.
     /// </summary>
+    /// <param name="truncated">
+    ///     Receives whether the fetch behind the snapshot hit the per-fetch cap,
+    ///     so the returned set may omit active alarms. Forwarded from the single
+    ///     atomic consumer read, and stamped onto every returned record as
+    ///     <c>ActiveAlarmSnapshot.FromTruncatedSnapshot</c>. Also returned
+    ///     separately because a snapshot of zero records still has to report it.
+    /// </param>
     /// <returns>The currently active alarm snapshots.</returns>
-    public IReadOnlyList<ActiveAlarmSnapshot> SnapshotActiveAlarms()
+    public IReadOnlyList<ActiveAlarmSnapshot> SnapshotActiveAlarms(out bool truncated)
     {
         if (disposed) throw new ObjectDisposedException(nameof(AlarmDispatcher));
-        // Read the truncation verdict before the snapshot, so a poll landing
-        // between the two can only widen the warning (a stale "truncated" over a
-        // complete snapshot), never narrow it into a false all-clear.
-        bool truncated = consumer.LastSnapshotTruncated;
-        IReadOnlyList<MxAlarmSnapshotRecord> records = consumer.SnapshotActiveAlarms();
+        // One consumer call yields the records and the verdict together, under a
+        // single acquisition of the consumer's snapshot lock. Reading them
+        // separately would let a poll interleave and pair a stale not-truncated
+        // verdict with a capped snapshot — a set that reads complete while
+        // missing actives. The atomicity is structural here, not a consequence of
+        // the STA happening to serialize the two calls.
+        IReadOnlyList<MxAlarmSnapshotRecord> records = consumer.SnapshotActiveAlarms(out truncated);
         if (records.Count == 0) return Array.Empty<ActiveAlarmSnapshot>();
         List<ActiveAlarmSnapshot> snapshots = new List<ActiveAlarmSnapshot>(records.Count);
         foreach (MxAlarmSnapshotRecord record in records)
@@ -172,14 +181,6 @@ public sealed class AlarmDispatcher : IDisposable
         return snapshots;
     }
 
-    /// <summary>
-    ///     Whether the consumer's most recent fetch hit the per-fetch cap, so
-    ///     the set <see cref="SnapshotActiveAlarms"/> returns may omit actives.
-    ///     Stamped onto the QueryActiveAlarms reply payload, which is the only
-    ///     carrier when the snapshot filters down to zero records.
-    /// </summary>
-    public bool LastSnapshotTruncated => !disposed && consumer.LastSnapshotTruncated;
-
     private void OnTransition(object? sender, MxAlarmTransitionEvent transition)
     {
         if (disposed) return;