refactor(engine): align namespaces with directory layout under OpenNest.Engine

All 132 OpenNest.Engine source files now declare namespaces matching
their nested directories: Jobs/, Jobs/Placement/, Jobs/Adapters/,
Fill/, RectanglePacking/, CirclePacking/, and engine-root types moved
from 'OpenNest' to 'OpenNest.Engine'. RootNamespace updated accordingly.
Consumers (Api, Console, Mcp, Benchmark, Training, desktop app, tests)
gained the explicit usings the move requires; CLAUDE.md updated.

OpenNest.Engine/Fill/BestCombination.cs

@@ -1,6 +1,6 @@
 using OpenNest.Math;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     internal record CombinationResult(bool Found, int Count1, int Count2);
 

OpenNest.Engine/Fill/ShrinkFiller.cs

@@ -3,7 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using OpenNest.Geometry;
-using OpenNest.RectanglePacking;
+using OpenNest.Engine.RectanglePacking;
 
 namespace OpenNest.Engine.Fill
 {