refactor(engine): share the private-plate Fill run between blocks and future callers

Move BlockCatalog's private-drawing Fill invocation (BestFitCache stabilization plus
the Default FillItem call) into PrivatePlateFill so the cutout fill can reuse it
without duplicating the cache handling. Behavior unchanged; Irregular filter 96/96.

OpenNest.Engine/Jobs/Placement/PrivatePlateFill.cs

@@ -0,0 +1,41 @@
+#nullable enable
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using OpenNest.Engine.BestFit;
+using OpenNest.Engine.Jobs.Adapters;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Jobs.Placement;
+
+/// <summary>
+/// Runs the Default Fill for one private drawing on a private plate, for callers that turn the
+/// result into job poses. The caller owns the drawing and must call
+/// <see cref="BestFitCache.Invalidate"/> for it when finished.
+/// </summary>
+internal static class PrivatePlateFill
+{
+    /// <param name="length">X extent of the plate and work area.</param>
+    /// <param name="width">Y extent of the plate and work area.</param>
+    /// <param name="quantity">Copies wanted; zero fills the whole area.</param>
+    internal static List<Part> Run(Drawing drawing, RotationPolicy rotation, double spacing,
+        double length, double width, int quantity, CancellationToken token)
+    {
+        var plate = new Plate(new Size(width, length)) { PartSpacing = spacing };
+        // The drawing is private: stabilize this cache entry before Fill's candidate pruning.
+        var fits = BestFitCache.GetOrCompute(drawing, plate.Size.Length, plate.Size.Width, spacing);
+        var sorted = fits.OrderBy(f => f.RotatedArea).ThenBy(f => f.Candidate.StrategyIndex)
+            .ThenBy(f => f.Candidate.Part2Rotation).ThenBy(f => f.Candidate.Part2Offset.X)
+            .ThenBy(f => f.Candidate.Part2Offset.Y).ThenBy(f => f.OptimalRotation).ToArray();
+        fits.Clear();
+        fits.AddRange(sorted);
+        return PlateFillService.FillItem("Default", plate, new NestItem
+        {
+            Drawing = drawing,
+            Quantity = quantity,
+            RotationStart = rotation.Start,
+            RotationEnd = rotation.End,
+            StepAngle = DrawingJobMapper.LegacyStep(rotation),
+        }, new Box(0, 0, length, width), null!, token);
+    }
+}

OpenNest.Engine/NestingEngines/Irregular/BlockCatalog.cs

@@ -77,24 +77,10 @@ internal sealed class BlockCatalog : IDisposable
     {
         if (!drawings.TryGetValue(type.Index, out var drawing))
             drawings[type.Index] = drawing = DrawingJobMapper.CreateDrawing(type.Part);
-        var plate = new Plate(new Size(rectangle.Width, rectangle.Length)) { PartSpacing = spacing };
         try
         {
-            // The drawing is private: stabilize this cache entry before Fill's candidate pruning.
-            var fits = BestFitCache.GetOrCompute(drawing, plate.Size.Length, plate.Size.Width, spacing);
-            var sorted = fits.OrderBy(f => f.RotatedArea).ThenBy(f => f.Candidate.StrategyIndex)
-                .ThenBy(f => f.Candidate.Part2Rotation).ThenBy(f => f.Candidate.Part2Offset.X)
-                .ThenBy(f => f.Candidate.Part2Offset.Y).ThenBy(f => f.OptimalRotation).ToArray();
-            fits.Clear();
-            fits.AddRange(sorted);
-            var members = PlateFillService.FillItem("Default", plate, new NestItem
-            {
-                Drawing = drawing,
-                Quantity = quantity,
-                RotationStart = type.Part.Rotation.Start,
-                RotationEnd = type.Part.Rotation.End,
-                StepAngle = DrawingJobMapper.LegacyStep(type.Part.Rotation),
-            }, new Box(0, 0, rectangle.Length, rectangle.Width), null!, token);
+            var members = PrivatePlateFill.Run(drawing, type.Part.Rotation, spacing,
+                rectangle.Length, rectangle.Width, quantity, token);
             token.ThrowIfCancellationRequested();
             return Resolve(type, members.Take(quantity).ToArray(), orientations[type.Index], spacing, token);
         }