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/NestProgress.cs

@@ -5,7 +5,7 @@ using System.ComponentModel;
 using System.Reflection;
 using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine
 {
     [AttributeUsage(AttributeTargets.Field)]
     internal class ShortNameAttribute(string name) : Attribute