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.Benchmark/Program.cs

@@ -5,6 +5,7 @@ using System.Linq;
 using OpenNest;
 using OpenNest.Benchmark;
 using OpenNest.Geometry;
+using OpenNest.Engine.Jobs;
 
 return BenchmarkConsole.Run(args);