feat: add AutoNester.Optimize post-pass and NfpNestEngine

Add Optimize method that re-places parts using NFP-based BLF, keeping
the result only if it improves density without losing parts. Fix
perimeter inflation to use correct offset side. Add NfpNestEngine
that wraps AutoNester for the registry. Register NFP engine.

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

OpenNest.Engine/NestEngineRegistry.cs

@@ -20,6 +20,10 @@ namespace OpenNest
             Register("Strip",
                 "Strip-based nesting for mixed-drawing layouts",
                 plate => new StripNestEngine(plate));
+
+            Register("NFP",
+                "NFP-based mixed-part nesting with simulated annealing",
+                plate => new NfpNestEngine(plate));
         }
 
         public static IReadOnlyList<NestEngineInfo> AvailableEngines => engines;