feat(ui): remember the selected Auto Nest engine

OpenNest.Data/EngineSelectionSettings.cs

@@ -0,0 +1,95 @@
+using System.Text.Json;
+
+namespace OpenNest.Data;
+
+/// <summary>
+/// Last-used Auto Nest engine, stored separately from nest/plate defaults.
+/// Loading does not resolve the name: the host must finish plug-in discovery first.
+/// </summary>
+public sealed class EngineSelectionSettings
+{
+    public const string DefaultEngineName = "Default";
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        PropertyNameCaseInsensitive = true,
+    };
+
+    public string EngineName { get; set; } = DefaultEngineName;
+
+    /// <summary>%APPDATA%\OpenNest\engine-selection.json.</summary>
+    public static string DefaultPath => Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+        "OpenNest", "engine-selection.json");
+
+    /// <summary>Missing, unreadable or corrupt settings safely use Default without writing.</summary>
+    public static EngineSelectionSettings Load(string path)
+    {
+        if (string.IsNullOrWhiteSpace(path) || !File.Exists(path))
+            return new();
+
+        try
+        {
+            var settings = JsonSerializer.Deserialize<EngineSelectionSettings>(
+                File.ReadAllText(path), JsonOptions) ?? new();
+            settings.EngineName = NormalizeName(settings.EngineName);
+            return settings;
+        }
+        catch (Exception ex) when (ex is JsonException or IOException or UnauthorizedAccessException)
+        {
+            return new();
+        }
+    }
+
+    /// <summary>
+    /// Resolves against the host's selectable engines AFTER plug-in loading. A missing engine
+    /// returns Default plus a status-bar warning, without replacing the saved preference.
+    /// Names use registry casing so desktop combo-box selection remains exact.
+    /// </summary>
+    public string Resolve(IEnumerable<string> availableEngineNames, out string? statusMessage)
+    {
+        ArgumentNullException.ThrowIfNull(availableEngineNames);
+        var requestedName = NormalizeName(EngineName);
+        var registeredName = availableEngineNames.FirstOrDefault(
+            name => string.Equals(name, requestedName, StringComparison.OrdinalIgnoreCase));
+        if (registeredName is not null)
+        {
+            statusMessage = null;
+            return registeredName;
+        }
+
+        statusMessage = $"Saved Auto Nest engine '{requestedName}' is unavailable. Using Default.";
+        return DefaultEngineName;
+    }
+
+    /// <summary>
+    /// Writes camelCase JSON, creating the parent directory and retrying IO collisions as
+    /// LocalJsonProvider does. The host handles a persistent write failure.
+    /// </summary>
+    public void Save(string path)
+    {
+        var json = JsonSerializer.Serialize(
+            new EngineSelectionSettings { EngineName = NormalizeName(EngineName) }, JsonOptions);
+        var directory = Path.GetDirectoryName(Path.GetFullPath(path));
+
+        for (var attempt = 0; attempt < 3; attempt++)
+        {
+            try
+            {
+                if (!string.IsNullOrEmpty(directory))
+                    Directory.CreateDirectory(directory);
+                File.WriteAllText(path, json);
+                return;
+            }
+            catch (IOException) when (attempt < 2)
+            {
+                Thread.Sleep(100);
+            }
+        }
+    }
+
+    private static string NormalizeName(string? name) =>
+        string.IsNullOrWhiteSpace(name) ? DefaultEngineName : name.Trim();
+}