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/Jobs/Adapters/DrawingJobMapper.cs

@@ -1,7 +1,7 @@
 using System;
 using OpenNest.CNC;
 
-namespace OpenNest;
+namespace OpenNest.Engine.Jobs.Adapters;
 
 /// <summary>Explicit-ID input mapping and exact supported-geometry reconstruction. Never retains caller objects.</summary>
 public static class DrawingJobMapper

OpenNest.Engine/Jobs/Adapters/LegacyPlateNesterAdapter.cs

@@ -2,7 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.Threading;
 
-namespace OpenNest;
+namespace OpenNest.Engine.Jobs.Adapters;
 
 /// <summary>
 /// A fresh private legacy plate/drawing/item graph for each call. Only returned poses cross the boundary;

OpenNest.Engine/Jobs/Adapters/NestResultMaterializer.cs

@@ -4,7 +4,7 @@ using System.Collections.ObjectModel;
 using System.Linq;
 using OpenNest.Geometry;
 
-namespace OpenNest;
+namespace OpenNest.Engine.Jobs.Adapters;
 
 /// <summary>A detached mutable domain nest plus explicit requirement identity (never inferred from names).</summary>
 public sealed class MaterializedNestResult