fix(api): set plate Material from request, add null guards in LoadAsync

- NestRunner now assigns Material to plates from request.Material
- NestResponse.LoadAsync uses descriptive exceptions instead of null-forgiving operators
- Fix pre-existing FillExtents.Fill signature mismatch (add bestFits parameter)

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

OpenNest.Api/NestResponse.cs

@@ -67,27 +67,30 @@ public class NestResponse
         using var zip = new ZipArchive(fs, ZipArchiveMode.Read);
 
         // Read request.json
-        var requestEntry = zip.GetEntry("request.json");
+        var requestEntry = zip.GetEntry("request.json")
+            ?? throw new InvalidOperationException("Missing request.json in .nestquote file");
         NestRequest request;
-        await using (var stream = requestEntry!.Open())
+        await using (var stream = requestEntry.Open())
         {
             request = await JsonSerializer.DeserializeAsync<NestRequest>(stream, JsonOptions);
         }
 
         // Read response.json
-        var responseEntry = zip.GetEntry("response.json");
+        var responseEntry = zip.GetEntry("response.json")
+            ?? throw new InvalidOperationException("Missing response.json in .nestquote file");
         JsonElement metricsJson;
-        await using (var stream = responseEntry!.Open())
+        await using (var stream = responseEntry.Open())
         {
             metricsJson = await JsonSerializer.DeserializeAsync<JsonElement>(stream, JsonOptions);
         }
 
         // Read embedded nest.nest via NestReader(Stream)
-        var nestEntry = zip.GetEntry("nest.nest");
+        var nestEntry = zip.GetEntry("nest.nest")
+            ?? throw new InvalidOperationException("Missing nest.nest in .nestquote file");
         Nest nest;
         using (var nestMs = new MemoryStream())
         {
-            await using (var stream = nestEntry!.Open())
+            await using (var stream = nestEntry.Open())
             {
                 await stream.CopyToAsync(nestMs);
             }

OpenNest.Api/NestRunner.cs

@@ -67,7 +67,8 @@ public static class NestRunner
             var plate = new Plate(request.SheetSize)
             {
                 Thickness = request.Thickness,
-                PartSpacing = request.Spacing
+                PartSpacing = request.Spacing,
+                Material = new Material(request.Material)
             };
 
             // Build items for this pass with remaining quantities

OpenNest.Engine/FillExtents.cs

@@ -25,7 +25,8 @@ namespace OpenNest
         public List<Part> Fill(Drawing drawing, double rotationAngle = 0,
             int plateNumber = 0,
             CancellationToken token = default,
-            IProgress<NestProgress> progress = null)
+            IProgress<NestProgress> progress = null,
+            List<Engine.BestFit.BestFitResult> bestFits = null)
         {
             var pair = BuildPair(drawing, rotationAngle);
             if (pair == null)