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.Api/NestResponse.cs

@@ -6,6 +6,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Threading.Tasks;
 using OpenNest.IO;
+using OpenNest.Engine.Jobs;
 
 namespace OpenNest.Api;
 

OpenNest.Api/NestRunner.cs

@@ -6,6 +6,9 @@ using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using OpenNest.IO;
+using OpenNest.Engine;
+using OpenNest.Engine.Jobs;
+using OpenNest.Engine.Jobs.Adapters;
 
 namespace OpenNest.Api;