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/NfpCache.cs

@@ -1,8 +1,8 @@
+using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Nfp
 {
     /// <summary>
     /// Caches computed No-Fit Polygons keyed by (DrawingA.Id, RotationA, DrawingB.Id, RotationB).