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);
     }
 }

OpenNest.Engine/Nfp/NfpCache.cs

@@ -14,6 +14,8 @@ namespace OpenNest.Engine.Nfp
         private readonly Dictionary<NfpKey, Polygon> cache = new Dictionary<NfpKey, Polygon>();
         private readonly Dictionary<int, Dictionary<double, Polygon>> polygonCache
             = new Dictionary<int, Dictionary<double, Polygon>>();
+        private readonly Dictionary<(int drawingId, double rotation), Polygon> ifpCache
+            = new Dictionary<(int drawingId, double rotation), Polygon>();
 
         /// <summary>
         /// Registers a pre-computed polygon for a drawing at a specific rotation.
@@ -28,6 +30,26 @@ namespace OpenNest.Engine.Nfp
             }
 
             rotations[rotation] = polygon;
+
+            // Clear IFP cache if a polygon is updated (though usually they aren't).
+            ifpCache.Remove((drawingId, rotation));
+        }
+
+        /// <summary>
+        /// Gets or computes the IFP for a drawing at a specific rotation within a work area.
+        /// </summary>
+        public Polygon GetIfp(int drawingId, double rotation, Box workArea)
+        {
+            if (ifpCache.TryGetValue((drawingId, rotation), out var ifp))
+                return ifp;
+
+            var polygon = GetPolygon(drawingId, rotation);
+            if (polygon == null)
+                return new Polygon();
+
+            ifp = InnerFitPolygon.Compute(workArea, polygon);
+            ifpCache[(drawingId, rotation)] = ifp;
+            return ifp;
         }
 
         /// <summary>

OpenNest.Engine/Nfp/PlacedPart.cs

@@ -0,0 +1,15 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Nfp
+{
+    /// <summary>
+    /// Represents a part that has been placed by the BLF algorithm.
+    /// </summary>
+    public class PlacedPart
+    {
+        public int DrawingId { get; set; }
+        public double Rotation { get; set; }
+        public Vector Position { get; set; }
+        public Drawing Drawing { get; set; }
+    }
+}

OpenNest.Engine/Nfp/SequenceEntry.cs

@@ -0,0 +1,24 @@
+namespace OpenNest.Engine.Nfp
+{
+    /// <summary>
+    /// An entry in a placement sequence — identifies which drawing to place and at what rotation.
+    /// </summary>
+    public readonly struct SequenceEntry
+    {
+        public int DrawingId { get; }
+        public double Rotation { get; }
+        public Drawing Drawing { get; }
+
+        public SequenceEntry(int drawingId, double rotation, Drawing drawing)
+        {
+            DrawingId = drawingId;
+            Rotation = rotation;
+            Drawing = drawing;
+        }
+
+        public SequenceEntry WithRotation(double rotation)
+        {
+            return new SequenceEntry(DrawingId, rotation, Drawing);
+        }
+    }
+}