feat: add IDataProvider interface and LocalJsonProvider with JSON file CRUD

One JSON file per machine named by GUID, stored in a configurable directory.
Supports save, load, list (as summaries), and delete with IO-error retry.

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

OpenNest.Tests/Data/LocalJsonProviderTests.cs

@@ -0,0 +1,163 @@
+using OpenNest.Data;
+
+namespace OpenNest.Tests.Data;
+
+public class LocalJsonProviderTests : IDisposable
+{
+    private readonly string _testDir;
+
+    public LocalJsonProviderTests()
+    {
+        _testDir = Path.Combine(Path.GetTempPath(), "OpenNestTests", Guid.NewGuid().ToString());
+        Directory.CreateDirectory(_testDir);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_testDir))
+            Directory.Delete(_testDir, true);
+    }
+
+    private LocalJsonProvider CreateProvider() => new(_testDir);
+
+    [Fact]
+    public void GetMachines_EmptyDirectory_ReturnsEmpty()
+    {
+        var provider = CreateProvider();
+        var result = provider.GetMachines();
+        Assert.Empty(result);
+    }
+
+    [Fact]
+    public void SaveMachine_ThenGetMachine_RoundTrips()
+    {
+        var provider = CreateProvider();
+        var machine = new MachineConfig
+        {
+            Name = "Test Laser",
+            Type = MachineType.Laser,
+            Units = UnitSystem.Inches,
+            Materials = new List<MaterialConfig>
+            {
+                new()
+                {
+                    Name = "Mild Steel",
+                    Grade = "A36",
+                    Density = 0.2836,
+                    Thicknesses = new List<ThicknessConfig>
+                    {
+                        new()
+                        {
+                            Value = 0.250,
+                            Kerf = 0.012,
+                            AssistGas = "O2",
+                            LeadIn = new LeadConfig { Type = "Arc", Length = 0.25, Angle = 90.0, Radius = 0.125 },
+                            LeadOut = new LeadConfig { Type = "Line", Length = 0.125 },
+                            CutOff = new CutOffConfig
+                            {
+                                PartClearance = 0.5,
+                                Overtravel = 0.25,
+                                Direction = "AwayFromOrigin",
+                                MinSegmentLength = 1.0
+                            },
+                            PlateSizes = new List<string> { "60x120", "48x96" }
+                        }
+                    }
+                }
+            }
+        };
+
+        provider.SaveMachine(machine);
+        var loaded = provider.GetMachine(machine.Id);
+
+        Assert.NotNull(loaded);
+        Assert.Equal("Test Laser", loaded.Name);
+        Assert.Equal(MachineType.Laser, loaded.Type);
+        Assert.Equal(UnitSystem.Inches, loaded.Units);
+        Assert.Single(loaded.Materials);
+
+        var mat = loaded.Materials[0];
+        Assert.Equal("Mild Steel", mat.Name);
+        Assert.Equal("A36", mat.Grade);
+        Assert.Equal(0.2836, mat.Density);
+        Assert.Single(mat.Thicknesses);
+
+        var thick = mat.Thicknesses[0];
+        Assert.Equal(0.250, thick.Value);
+        Assert.Equal(0.012, thick.Kerf);
+        Assert.Equal("O2", thick.AssistGas);
+        Assert.Equal("Arc", thick.LeadIn.Type);
+        Assert.Equal(0.25, thick.LeadIn.Length);
+        Assert.Equal("Line", thick.LeadOut.Type);
+        Assert.Equal(0.125, thick.LeadOut.Length);
+        Assert.Equal(0.5, thick.CutOff.PartClearance);
+        Assert.Equal("AwayFromOrigin", thick.CutOff.Direction);
+        Assert.Equal(new List<string> { "60x120", "48x96" }, thick.PlateSizes);
+    }
+
+    [Fact]
+    public void GetMachines_ReturnsSummaries()
+    {
+        var provider = CreateProvider();
+        var m1 = new MachineConfig { Name = "Laser A" };
+        var m2 = new MachineConfig { Name = "Plasma B" };
+
+        provider.SaveMachine(m1);
+        provider.SaveMachine(m2);
+        var summaries = provider.GetMachines();
+
+        Assert.Equal(2, summaries.Count);
+        Assert.Contains(summaries, s => s.Name == "Laser A" && s.Id == m1.Id);
+        Assert.Contains(summaries, s => s.Name == "Plasma B" && s.Id == m2.Id);
+    }
+
+    [Fact]
+    public void GetMachine_NotFound_ReturnsNull()
+    {
+        var provider = CreateProvider();
+        var result = provider.GetMachine(Guid.NewGuid());
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void DeleteMachine_RemovesFile()
+    {
+        var provider = CreateProvider();
+        var machine = new MachineConfig { Name = "To Delete" };
+        provider.SaveMachine(machine);
+
+        provider.DeleteMachine(machine.Id);
+
+        Assert.Null(provider.GetMachine(machine.Id));
+        Assert.Empty(provider.GetMachines());
+    }
+
+    [Fact]
+    public void SaveMachine_OverwritesExisting()
+    {
+        var provider = CreateProvider();
+        var machine = new MachineConfig { Name = "Original" };
+        provider.SaveMachine(machine);
+
+        machine.Name = "Updated";
+        provider.SaveMachine(machine);
+
+        var loaded = provider.GetMachine(machine.Id);
+        Assert.NotNull(loaded);
+        Assert.Equal("Updated", loaded.Name);
+        Assert.Single(provider.GetMachines());
+    }
+
+    [Fact]
+    public void SaveMachine_PreservesSchemaVersion()
+    {
+        var provider = CreateProvider();
+        var machine = new MachineConfig { Name = "Versioned", SchemaVersion = 1 };
+
+        provider.SaveMachine(machine);
+        var loaded = provider.GetMachine(machine.Id);
+
+        Assert.NotNull(loaded);
+        Assert.Equal(1, loaded.SchemaVersion);
+    }
+}