refactor: extract IDistanceComputer with CPU and GPU implementations

Extract distance computation from RotationSlideStrategy into a pluggable
IDistanceComputer interface. CpuDistanceComputer adds leading-face vertex
culling (~50% fewer rays per direction) with early exit on overlap.
GpuDistanceComputer wraps ISlideComputer with Line-to-flat-array conversion.
SlideOffset struct uses direction vectors (DirX/DirY) instead of PushDirection.
SpatialQuery.RayEdgeDistance(dirX,dirY) made public for CPU path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/BestFit/IDistanceComputer.cs

@@ -0,0 +1,13 @@
+using OpenNest.Geometry;
+using System.Collections.Generic;
+
+namespace OpenNest.Engine.BestFit
+{
+    public interface IDistanceComputer
+    {
+        double[] ComputeDistances(
+            List<Line> stationaryLines,
+            List<Line> movingTemplateLines,
+            SlideOffset[] offsets);
+    }
+}