refactor: extract PlacedPart/SequenceEntry types, add IFP caching

Move PlacedPart to its own file. Replace tuple-based sequences with
SequenceEntry struct for clarity. Add IProgress parameter to
INestOptimizer. Add IFP caching to NfpCache to avoid recomputing
inner fit polygons for the same drawing/rotation/workArea.

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

OpenNest.Engine/Nfp/INestOptimizer.cs

@@ -1,5 +1,6 @@
 using OpenNest.Engine.Fill;
 using OpenNest.Geometry;
+using System;
 using System.Collections.Generic;
 using System.Threading;
 
@@ -11,9 +12,9 @@ namespace OpenNest.Engine.Nfp
     public class OptimizationResult
     {
         /// <summary>
-        /// The best sequence found: (drawingId, rotation, drawing) tuples in placement order.
+        /// The best placement sequence found.
         /// </summary>
-        public List<(int drawingId, double rotation, Drawing drawing)> Sequence { get; set; }
+        public List<SequenceEntry> Sequence { get; set; }
 
         /// <summary>
         /// The score achieved by the best sequence.
@@ -34,6 +35,7 @@ namespace OpenNest.Engine.Nfp
     {
         OptimizationResult Optimize(List<NestItem> items, Box workArea, NfpCache cache,
             Dictionary<int, List<double>> candidateRotations,
+            IProgress<NestProgress> progress = null,
             CancellationToken cancellation = default);
     }
 }