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/OpcUaAlarmMapper.cs

@@ -65,4 +65,40 @@ public static class OpcUaAlarmMapper
         null or "Unshelved" => AlarmShelveState.Unshelved,
         _ => AlarmShelveState.OneShotShelved
     };
+
+    /// <summary>
+    /// Picks a representative display-only limit value from the four standard
+    /// <c>LimitAlarmType</c> set-point fields (HighHighLimit, HighLimit, LowLimit,
+    /// LowLowLimit) returned by the OPC UA event SelectClause.
+    ///
+    /// <para>
+    /// The fields are absent (null raw value) on non-limit alarm types (discrete,
+    /// off-normal, etc.).  When present, the first non-null value is returned in
+    /// priority order: HighHigh → High → Low → LowLow.  The caller may use
+    /// <c>AlarmTypeName</c> or <c>ConditionName</c> to determine which specific
+    /// limit is active; this method intentionally returns the coarsest useful value
+    /// for the common single-limit case without requiring callers to understand the
+    /// OPC UA limit hierarchy.
+    /// </para>
+    /// </summary>
+    /// <param name="highHighRaw">Raw HighHighLimit field value (null when absent).</param>
+    /// <param name="highRaw">Raw HighLimit field value (null when absent).</param>
+    /// <param name="lowRaw">Raw LowLimit field value (null when absent).</param>
+    /// <param name="lowLowRaw">Raw LowLowLimit field value (null when absent).</param>
+    /// <returns>
+    /// A formatted string representation of the first non-null limit value, or an
+    /// empty string when all four fields are absent (non-limit alarm type).
+    /// </returns>
+    public static string PickLimitValue(object? highHighRaw, object? highRaw, object? lowRaw, object? lowLowRaw)
+    {
+        // Standard OPC UA LimitAlarmType limit values are numeric (Double/Float/Int).
+        // Convert with InvariantCulture so the decimal separator is always '.' regardless
+        // of the server's locale.
+        foreach (var raw in new[] { highHighRaw, highRaw, lowRaw, lowLowRaw })
+        {
+            if (raw is not null)
+                return Convert.ToString(raw, System.Globalization.CultureInfo.InvariantCulture) ?? "";
+        }
+        return "";
+    }
 }