fix(alarms): Severity runs one direction, higher = more urgent; raw priority kept (#129)

AlarmConditionState.Severity mixed two conventions: OPC UA rows carried Part 9
Severity (higher = more severe) while MxGateway native alarms and computed
alarms carried a raw AVEVA-style priority (1 = most urgent). Every consumer
ranks higher as worse, so Worst Severity, the severity filter/sort and the
site event-log Error threshold picked the LEAST urgent MxGateway/computed
alarms.

- AlarmPriorityBands: the OtOpcUa GalaxyAlarmPriority table (lmxopcua#887,
  b61f3e05), value for value: 1-250 -> 1000, 251-500 -> 750, 501-750 -> 500,
  751-999 -> 250; <= 0 clamps to Critical, > 999 to Low.
- MxGatewayAlarmMapper bands the gateway severity (= AVEVA priority);
  AlarmConditionStateFactory.ForComputed bands PriorityLevel. OPC UA unchanged.
- AlarmConditionState gains trailing Priority (int?): the raw source number,
  exactly the pre-#129 Severity. AlarmStateChanged.Priority and the wire
  AlarmStateUpdate.priority keep that meaning (no proto change); central derives
  Severity by kind (SeverityFromPriority), so an older site decodes correctly.
- ScriptAlarm gains trailing Priority. The native MES 900-999 band and the MES
  API Severity/MinSeverity/MaxSeverity key off Priority; rig seed scripts moved
  from a.Severity to a.Priority, so MES output is unchanged.
- AlarmActor raise logs Error when the banded severity >= 700 (same threshold
  as NativeAlarmActor): priority 1-500 Error, 501+ Warning (was raw >= 700).
- NativeAlarmActor upgrades pre-#129 native_alarm_state rows on rehydrate.
- Debug snapshot decode restores Condition.Priority from the row's priority.
- Docs: Commons, DCL, SiteRuntime, Communication, CentralUI, InboundAPI,
  HighLevelReqs, MES alarm-status plan, CLAUDE.md.

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/SiteCommandSamples.cs

@@ -301,7 +301,7 @@ internal static class SiteCommandSamples
             Level = AlarmLevel.None,
             Message = "Level high",
             Kind = AlarmKind.NativeOpcUa,
-            Condition = new AlarmConditionState(true, false, false, AlarmShelveState.OneShotShelved, true, 850),
+            Condition = new AlarmConditionState(true, false, false, AlarmShelveState.OneShotShelved, true, 850, Priority: 850),
             SourceReference = "Tank01.Level.HiHi",
             AlarmTypeName = "AnalogLimitAlarm.HiHi",
             Category = "Process",