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/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);
+        }
+    }
+}