feat(streaming): carry AckTime on DebugAlarmStateDto (21) (ScadaBridge#84)

The live AlarmStateUpdate already carried ack_time (24) but the Debug View
snapshot DTO did not, so snapshot-seeded alarm rows showed a null ack time
until the next live update. Additive google.protobuf.Timestamp ack_time = 21
(absent => null, including from a pre-#84 site), vendored C# regenerated with
docker/regen-proto.sh sitecommand, mapped both ways in SiteCommandDtoMapper.
The native golden sample now sets AckTime (the computed one leaves it null),
so the round-trip golden pins both the present and the absent decode.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Streaming/AlarmStateChanged.cs

@@ -85,6 +85,11 @@ public record AlarmStateChanged(
     /// null while a native condition is unacknowledged; cleared on re-raise, because a
     /// re-raise transition carries <c>Acknowledged = false</c>.
     /// </para>
+    ///
+    /// <para>
+    /// On the wire: <c>AlarmStateUpdate</c> field 24 (live stream) and
+    /// <c>DebugAlarmStateDto</c> field 21 (debug snapshot, Gitea #84) — absent decodes to null.
+    /// </para>
     /// </summary>
     public DateTimeOffset? AckTime { get; init; }