chore(core): flatten Historian sub-namespace to Core.Abstractions root

Four continuous-historization types (IHistorizationOutbox, HistorizationOutboxEntry,
IHistorianValueWriter/HistorizationValue, HistorizationCommitMode) drifted into a
ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian sub-namespace while the other six files
in the same folder correctly use the root namespace. This violated decision #59's
flat-public-surface rule and left InterfaceIndependenceTests.AllPublicTypes_LiveInRootNamespace
red on master (pre-existing, predates round-2 remediation).

Move all four to the root namespace and drop/retarget the now-redundant
'using ...Core.Abstractions.Historian;' in the ~11 consumers. No behavioral change.

Verified: Core.Abstractions.Tests 129/129 (was 128/129), Runtime.Tests Historian+DI 72/72,
Gateway driver unit 102/102, full solution build 0 errors.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ContinuousHistorizationMetrics.cs

@@ -1,5 +1,5 @@
 using ZB.MOM.WW.OtOpcUa.Commons.Observability;
-using ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Historian;
 

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ContinuousHistorizationRecorder.cs

@@ -1,7 +1,6 @@
 using Akka.Actor;
 using Akka.Event;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
-using ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
 using ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Historian;

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/ServiceCollectionExtensions.cs

@@ -11,7 +11,6 @@ using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
 using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
 using ZB.MOM.WW.OtOpcUa.Configuration;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
-using ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
 using ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian;
 using ZB.MOM.WW.OtOpcUa.Core.Scripting;
 using ZB.MOM.WW.OtOpcUa.Core.VirtualTags;