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/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/HistorizationCommitMode.cs

@@ -1,4 +1,4 @@
-namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 /// <summary>
 ///     Per-append durability cadence for the historization outbox. Local to the OtOpcUa abstraction

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/HistorizationOutboxEntry.cs

@@ -1,4 +1,4 @@
-namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 /// <summary>
 ///     One durable record buffered by the continuous-historization outbox before it is written to

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/IHistorianValueWriter.cs

@@ -1,4 +1,4 @@
-namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 /// <summary>
 ///     One numeric sample the continuous-historization recorder drains to the historian's SQL

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/IHistorizationOutbox.cs

@@ -1,4 +1,4 @@
-namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions.Historian;
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 /// <summary>
 ///     Durable, crash-safe FIFO buffer the continuous-historization recorder appends sampled values