feat: add ISlideComputer interface and GPU implementation

ISlideComputer abstracts batched directional-distance computation so GPU
implementations can process all slide offsets in a single kernel launch.
GpuSlideComputer uses ILGPU with prepared edge data (precomputed inverse
deltas and min/max bounds) and caches stationary/moving buffers across
calls. GpuEvaluatorFactory exposes a singleton factory method.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Engine/BestFit/ISlideComputer.cs

@@ -0,0 +1,38 @@
+using System;
+
+namespace OpenNest.Engine.BestFit
+{
+    /// <summary>
+    /// Batches directional-distance computations for multiple offset positions.
+    /// GPU implementations can process all offsets in a single kernel launch.
+    /// </summary>
+    public interface ISlideComputer : IDisposable
+    {
+        /// <summary>
+        /// Computes the minimum directional distance for each offset position.
+        /// </summary>
+        /// <param name="stationarySegments">Flat array [x1,y1,x2,y2, ...] for stationary edges.</param>
+        /// <param name="stationaryCount">Number of line segments in stationarySegments.</param>
+        /// <param name="movingTemplateSegments">Flat array [x1,y1,x2,y2, ...] for moving edges at origin.</param>
+        /// <param name="movingCount">Number of line segments in movingTemplateSegments.</param>
+        /// <param name="offsets">Flat array [dx,dy, dx,dy, ...] of translation offsets.</param>
+        /// <param name="offsetCount">Number of offset positions.</param>
+        /// <param name="direction">Push direction.</param>
+        /// <returns>Array of minimum distances, one per offset position.</returns>
+        double[] ComputeBatch(
+            double[] stationarySegments, int stationaryCount,
+            double[] movingTemplateSegments, int movingCount,
+            double[] offsets, int offsetCount,
+            PushDirection direction);
+
+        /// <summary>
+        /// Computes minimum directional distance for offsets with per-offset directions.
+        /// Uploads segment data once for all offsets, reducing GPU round-trips.
+        /// </summary>
+        double[] ComputeBatchMultiDir(
+            double[] stationarySegments, int stationaryCount,
+            double[] movingTemplateSegments, int movingCount,
+            double[] offsets, int offsetCount,
+            int[] directions);
+    }
+}