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/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; }
+    }
+}