refactor(engine): move NFP code to OpenNest.Engine.Nfp namespace

Move AutoNester, BottomLeftFill, NfpCache, SimulatedAnnealing,
and INestOptimizer/NestResult to OpenNest.Engine.Nfp. These are
not yet integrated into the engine registry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Nfp/AutoNester.cs

@@ -1,13 +1,12 @@
-using System;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+using OpenNest.Math;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading;
-using OpenNest.Converters;
-using OpenNest.Geometry;
-using OpenNest.Math;
 
-namespace OpenNest
+namespace OpenNest.Engine.Nfp
 {
     /// <summary>
     /// Mixed-part geometry-aware nesting using NFP-based collision avoidance