feat(persistence): add ConfigStore with atomic JSON writes and corruption-safe load

src/TeamsISO.Engine/Persistence/ConfigStore.cs

@@ -0,0 +1,59 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Microsoft.Extensions.Logging;
+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Persistence;
+
+/// <summary>
+/// Loads and saves <see cref="EngineConfig"/> as JSON.
+/// Writes are atomic: serialize to a temp file in the same directory, then <c>File.Move</c> with overwrite.
+/// </summary>
+public sealed class ConfigStore
+{
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        Converters = { new JsonStringEnumConverter() }
+    };
+
+    private readonly string _path;
+    private readonly ILogger<ConfigStore> _logger;
+
+    public ConfigStore(string path, ILogger<ConfigStore> logger)
+    {
+        _path = path;
+        _logger = logger;
+    }
+
+    public EngineConfig Load()
+    {
+        if (!File.Exists(_path))
+        {
+            _logger.LogInformation("Config file not found at {Path}; using defaults.", _path);
+            return EngineConfig.Default;
+        }
+
+        try
+        {
+            var json = File.ReadAllText(_path);
+            var loaded = JsonSerializer.Deserialize<EngineConfig>(json, Options);
+            return loaded ?? EngineConfig.Default;
+        }
+        catch (JsonException ex)
+        {
+            _logger.LogWarning(ex, "Config file at {Path} is not valid JSON; using defaults.", _path);
+            return EngineConfig.Default;
+        }
+    }
+
+    public void Save(EngineConfig config)
+    {
+        Directory.CreateDirectory(Path.GetDirectoryName(_path)!);
+        var temp = _path + ".tmp";
+        var json = JsonSerializer.Serialize(config, Options);
+        File.WriteAllText(temp, json);
+        File.Move(temp, _path, overwrite: true);
+    }
+}

src/tests/TeamsISO.Engine.Tests/Persistence/ConfigStoreTests.cs

@@ -0,0 +1,77 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using TeamsISO.Engine.Domain;
+using TeamsISO.Engine.Persistence;
+
+namespace TeamsISO.Engine.Tests.Persistence;
+
+public class ConfigStoreTests : IDisposable
+{
+    private readonly string _dir;
+
+    public ConfigStoreTests()
+    {
+        _dir = Path.Combine(Path.GetTempPath(), $"teamsiso-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_dir);
+    }
+
+    public void Dispose() => Directory.Delete(_dir, recursive: true);
+
+    private ConfigStore NewStore() =>
+        new(Path.Combine(_dir, "config.json"), NullLogger<ConfigStore>.Instance);
+
+    [Fact]
+    public void Load_WhenFileMissing_ReturnsDefault()
+    {
+        var store = NewStore();
+        var config = store.Load();
+        config.Should().Be(EngineConfig.Default);
+    }
+
+    [Fact]
+    public void SaveThenLoad_RoundTripsExactly()
+    {
+        var store = NewStore();
+        var input = new EngineConfig(
+            new FrameProcessingSettings(TargetFramerate.Fps59_94, TargetResolution.R720p,
+                AspectMode.Letterbox, AudioMode.Isolated),
+            new[]
+            {
+                new IsoAssignment(Guid.NewGuid(), IsEnabled: true, CustomOutputName: "TEAMSISO_A"),
+                new IsoAssignment(Guid.NewGuid(), IsEnabled: false, CustomOutputName: null),
+            });
+
+        store.Save(input);
+        var loaded = store.Load();
+
+        loaded.Should().BeEquivalentTo(input);
+    }
+
+    [Fact]
+    public void Load_WhenFileCorrupt_ReturnsDefault()
+    {
+        var path = Path.Combine(_dir, "config.json");
+        File.WriteAllText(path, "not valid json {{{");
+
+        var store = NewStore();
+        var config = store.Load();
+        config.Should().Be(EngineConfig.Default);
+    }
+
+    [Fact]
+    public void Save_WritesAtomically_NoPartialFileVisible()
+    {
+        var store = NewStore();
+        var path = Path.Combine(_dir, "config.json");
+
+        store.Save(EngineConfig.Default);
+        var firstWrite = File.ReadAllText(path);
+
+        store.Save(new EngineConfig(
+            FrameProcessingSettings.Default with { Framerate = TargetFramerate.Fps60 },
+            Array.Empty<IsoAssignment>()));
+
+        File.Exists(path).Should().BeTrue();
+        File.ReadAllText(path).Should().NotBe(firstWrite);
+        Directory.GetFiles(_dir).Should().HaveCount(1);
+    }
+}