feat(dcl): populate obtainable NativeAlarmTransition fields from OPC UA and MxGateway (#27, M2.13)

OPC UA (RealOpcUaClient):
- Append 5 new SelectClauses at indices 13–17 (never renumber 0–12):
  - 13: AlarmConditionType/ActiveState/TransitionTime → OriginalRaiseTime
  - 14–17: LimitAlarmType HighHighLimit/HighLimit/LowLimit/LowLowLimit → LimitValue
- New OpcUaAlarmMapper.PickLimitValue helper: first non-null in HiHi→Hi→Lo→LoLo
  priority order, InvariantCulture-formatted; empty string for non-limit alarm types.
- HandleAlarmEvent reads new indices with fields.Count > N guards; hard minimum (6)
  unchanged so base ConditionType events still process without the limit fields.
- Document unavailable-by-protocol fields (Category, Description, OperatorUser,
  CurrentValue) inline in BuildAlarmEventFilter and HandleAlarmEvent.

MxGateway (MxGatewayAlarmMapper):
- MapTransition: CurrentValue and LimitValue now populated via MxValueToString
  (uses MxValueExtensions.ToClrValue + InvariantCulture) from OnAlarmTransitionEvent
  proto fields current_value/limit_value.
- MapSnapshot: same — populated from ActiveAlarmSnapshot.current_value/limit_value.
- MxValueToString helper (internal): null-safe MxValue → string conversion.

Tests (17 new, 40 total pass):
- OpcUaAlarmMapperTests: PickLimitValue priority, InvariantCulture, all-null case.
- MxGatewayAlarmMapperTests: CurrentValue/LimitValue populate from double/string
  MxValue; absent fields yield empty strings.
- RealOpcUaClientAlarmFilterTests: index alignment assertions (count=18, per-index
  TypeDefinitionId+BrowsePath), regression guard on existing indices 0–12.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/MxGatewayAlarmMapper.cs

@@ -1,3 +1,5 @@
+using System.Globalization;
+using ZB.MOM.WW.MxGateway.Client;
 using ZB.MOM.WW.MxGateway.Contracts.Proto;
 using ProtoConditionState = ZB.MOM.WW.MxGateway.Contracts.Proto.AlarmConditionState;
 using ProtoTransitionKind = ZB.MOM.WW.MxGateway.Contracts.Proto.AlarmTransitionKind;
@@ -67,6 +69,19 @@ public static class MxGatewayAlarmMapper
             Shelve: AlarmShelveState.Unshelved, Suppressed: false, Severity: NormalizeSeverity(severity));
     }
 
+    /// <summary>
+    /// Converts an <see cref="MxValue"/> union to a display-only string using
+    /// <see cref="MxValueExtensions.ToClrValue"/> and invariant culture formatting,
+    /// so numeric values always use '.' as the decimal separator.  Null or unset
+    /// values produce an empty string.
+    /// </summary>
+    internal static string MxValueToString(MxValue? mxVal)
+    {
+        if (mxVal is null) return "";
+        var clr = mxVal.ToClrValue();
+        return clr is null ? "" : Convert.ToString(clr, CultureInfo.InvariantCulture) ?? "";
+    }
+
     /// <summary>Maps a live <see cref="OnAlarmTransitionEvent"/> to a transition.</summary>
     /// <param name="body">The gateway alarm transition event proto message to map.</param>
     /// <returns>The protocol-neutral <see cref="NativeAlarmTransition"/>.</returns>
@@ -83,8 +98,8 @@ public static class MxGatewayAlarmMapper
         OperatorComment: body.OperatorComment,
         OriginalRaiseTime: body.OriginalRaiseTimestamp?.ToDateTimeOffset(),
         TransitionTime: body.TransitionTimestamp?.ToDateTimeOffset() ?? DateTimeOffset.UtcNow,
-        CurrentValue: "",
-        LimitValue: "");
+        CurrentValue: MxValueToString(body.CurrentValue),
+        LimitValue: MxValueToString(body.LimitValue));
 
     /// <summary>The end-of-snapshot sentinel transition (no condition payload).</summary>
     /// <returns>A <see cref="NativeAlarmTransition"/> with <c>AlarmTransitionKind.SnapshotComplete</c>.</returns>
@@ -109,6 +124,6 @@ public static class MxGatewayAlarmMapper
         OperatorComment: snapshot.OperatorComment,
         OriginalRaiseTime: snapshot.OriginalRaiseTimestamp?.ToDateTimeOffset(),
         TransitionTime: snapshot.LastTransitionTimestamp?.ToDateTimeOffset() ?? DateTimeOffset.UtcNow,
-        CurrentValue: "",
-        LimitValue: "");
+        CurrentValue: MxValueToString(snapshot.CurrentValue),
+        LimitValue: MxValueToString(snapshot.LimitValue));
 }