feat(api): add NestResponse with SaveAsync/LoadAsync

Adds NestResponse type to OpenNest.Api with SaveAsync/LoadAsync for .nestquote format — a ZIP containing request.json, response.json (metrics), and an embedded nest.nest.

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

OpenNest.Tests/Api/NestResponsePersistenceTests.cs

@@ -0,0 +1,63 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using OpenNest.Api;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Api;
+
+public class NestResponsePersistenceTests
+{
+    [Fact]
+    public async Task SaveAsync_LoadAsync_RoundTrips()
+    {
+        var nest = new Nest("test-nest");
+        var plate = new Plate(new Size(60, 120));
+        nest.Plates.Add(plate);
+
+        var request = new NestRequest
+        {
+            Parts = [new NestRequestPart { DxfPath = "test.dxf", Quantity = 5 }],
+            SheetSize = new Size(60, 120),
+            Material = "Steel",
+            Thickness = 0.125,
+            Spacing = 0.1
+        };
+
+        var original = new NestResponse
+        {
+            SheetCount = 1,
+            Utilization = 0.75,
+            CutTime = TimeSpan.FromMinutes(12.5),
+            Elapsed = TimeSpan.FromSeconds(3.2),
+            Nest = nest,
+            Request = request
+        };
+
+        var path = Path.Combine(Path.GetTempPath(), $"test-{Guid.NewGuid()}.nestquote");
+
+        try
+        {
+            await original.SaveAsync(path);
+            var loaded = await NestResponse.LoadAsync(path);
+
+            Assert.Equal(original.SheetCount, loaded.SheetCount);
+            Assert.Equal(original.Utilization, loaded.Utilization, precision: 4);
+            Assert.Equal(original.CutTime, loaded.CutTime);
+            Assert.Equal(original.Elapsed, loaded.Elapsed);
+
+            Assert.Equal(original.Request.Material, loaded.Request.Material);
+            Assert.Equal(original.Request.Thickness, loaded.Request.Thickness);
+            Assert.Equal(original.Request.Parts.Count, loaded.Request.Parts.Count);
+            Assert.Equal(original.Request.Parts[0].DxfPath, loaded.Request.Parts[0].DxfPath);
+            Assert.Equal(original.Request.Parts[0].Quantity, loaded.Request.Parts[0].Quantity);
+
+            Assert.NotNull(loaded.Nest);
+            Assert.Single(loaded.Nest.Plates);
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+}