feat: add embedded CL-980 default config with first-run EnsureDefaults

Embeds CL-980.json as a resource in OpenNest.Data and adds EnsureDefaults()
to LocalJsonProvider, which seeds the machines directory on first run when empty.

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

OpenNest.Tests/Data/DefaultConfigTests.cs

@@ -0,0 +1,75 @@
+using OpenNest.Data;
+
+namespace OpenNest.Tests.Data;
+
+public class DefaultConfigTests : IDisposable
+{
+    private readonly string _testDir;
+
+    public DefaultConfigTests()
+    {
+        _testDir = Path.Combine(Path.GetTempPath(), "OpenNestTests", Guid.NewGuid().ToString());
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_testDir))
+            Directory.Delete(_testDir, true);
+    }
+
+    [Fact]
+    public void EnsureDefaults_EmptyDirectory_CopiesDefaultConfig()
+    {
+        var provider = new LocalJsonProvider(_testDir);
+        provider.EnsureDefaults();
+        var machines = provider.GetMachines();
+        Assert.Single(machines);
+        Assert.Equal("CL-980", machines[0].Name);
+    }
+
+    [Fact]
+    public void EnsureDefaults_ExistingFiles_DoesNotCopy()
+    {
+        var provider = new LocalJsonProvider(_testDir);
+        var existing = new MachineConfig { Name = "My Machine" };
+        provider.SaveMachine(existing);
+
+        provider.EnsureDefaults();
+
+        var machines = provider.GetMachines();
+        Assert.Single(machines);
+        Assert.Equal("My Machine", machines[0].Name);
+    }
+
+    [Fact]
+    public void DefaultConfig_HasValidStructure()
+    {
+        var provider = new LocalJsonProvider(_testDir);
+        provider.EnsureDefaults();
+
+        var machines = provider.GetMachines();
+        var config = provider.GetMachine(machines[0].Id);
+
+        Assert.NotNull(config);
+        Assert.Equal(1, config.SchemaVersion);
+        Assert.Equal(MachineType.Laser, config.Type);
+        Assert.Equal(UnitSystem.Inches, config.Units);
+        Assert.NotEmpty(config.Materials);
+
+        foreach (var material in config.Materials)
+        {
+            Assert.False(string.IsNullOrWhiteSpace(material.Name));
+            Assert.NotEmpty(material.Thicknesses);
+
+            foreach (var thickness in material.Thicknesses)
+            {
+                Assert.True(thickness.Value > 0);
+                Assert.True(thickness.Kerf > 0);
+                Assert.False(string.IsNullOrWhiteSpace(thickness.AssistGas));
+                Assert.NotNull(thickness.LeadIn);
+                Assert.NotNull(thickness.LeadOut);
+                Assert.NotNull(thickness.CutOff);
+            }
+        }
+    }
+}