feat: add xUnit test project with fill and remnant fill tests

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

TestData.cs

@@ -0,0 +1,64 @@
+using OpenNest.IO;
+
+namespace OpenNest.Test;
+
+public static class TestData
+{
+    private static readonly string? BasePath = ResolveBasePath();
+
+    public static bool IsAvailable => BasePath != null;
+
+    public static string SkipReason =>
+        "Test data not found. Fixture .zip files should be in the repo root.";
+
+    public static string GetPath(string filename)
+    {
+        if (BasePath == null)
+            throw new InvalidOperationException(SkipReason);
+
+        var path = Path.Combine(BasePath, filename);
+
+        if (!File.Exists(path))
+            throw new FileNotFoundException($"Test fixture not found: {path}");
+
+        return path;
+    }
+
+    public static Nest LoadNest(string filename)
+    {
+        var reader = new NestReader(GetPath(filename));
+        return reader.Read();
+    }
+
+    public static Plate CleanPlateFrom(Plate reference)
+    {
+        var plate = new Plate();
+        plate.Size = reference.Size;
+        plate.PartSpacing = reference.PartSpacing;
+        plate.EdgeSpacing = reference.EdgeSpacing;
+        plate.Quadrant = reference.Quadrant;
+        return plate;
+    }
+
+    private static string? ResolveBasePath()
+    {
+        // Walk up from bin/Debug/net8.0-windows to find the repo root
+        // (where the .zip fixture files live alongside the csproj).
+        var dir = AppContext.BaseDirectory;
+
+        for (var i = 0; i < 6; i++)
+        {
+            var parent = Directory.GetParent(dir);
+
+            if (parent == null)
+                break;
+
+            dir = parent.FullName;
+
+            if (File.Exists(Path.Combine(dir, "OpenNest.Test.csproj")))
+                return dir;
+        }
+
+        return null;
+    }
+}