feat(core): add PluginDiscovery directory scan

src/AcDream.Core/Plugins/PluginDiscovery.cs

@@ -0,0 +1,38 @@
+namespace AcDream.Core.Plugins;
+
+public sealed record PluginDiscoveryResult(
+    string PluginDirectory,
+    PluginManifest? Manifest,
+    string? Error)
+{
+    public bool Success => Manifest is not null && Error is null;
+}
+
+public static class PluginDiscovery
+{
+    public static IReadOnlyList<PluginDiscoveryResult> Scan(string pluginsRootDirectory)
+    {
+        if (!Directory.Exists(pluginsRootDirectory))
+            return Array.Empty<PluginDiscoveryResult>();
+
+        var results = new List<PluginDiscoveryResult>();
+        foreach (var subdir in Directory.EnumerateDirectories(pluginsRootDirectory))
+        {
+            var manifestPath = Path.Combine(subdir, "plugin.json");
+            if (!File.Exists(manifestPath))
+                continue;
+
+            try
+            {
+                var json = File.ReadAllText(manifestPath);
+                var manifest = PluginManifest.Parse(json);
+                results.Add(new PluginDiscoveryResult(subdir, manifest, null));
+            }
+            catch (Exception ex)
+            {
+                results.Add(new PluginDiscoveryResult(subdir, null, ex.Message));
+            }
+        }
+        return results;
+    }
+}

tests/AcDream.Core.Tests/Plugins/PluginDiscoveryTests.cs

@@ -0,0 +1,91 @@
+using AcDream.Core.Plugins;
+
+namespace AcDream.Core.Tests.Plugins;
+
+public class PluginDiscoveryTests : IDisposable
+{
+    private readonly string _tempDir;
+
+    public PluginDiscoveryTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), "acdream-test-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(_tempDir);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+            Directory.Delete(_tempDir, recursive: true);
+    }
+
+    private string WriteManifest(string folder, string json)
+    {
+        var dir = Path.Combine(_tempDir, folder);
+        Directory.CreateDirectory(dir);
+        var path = Path.Combine(dir, "plugin.json");
+        File.WriteAllText(path, json);
+        return dir;
+    }
+
+    private const string ValidManifest = """
+    {
+      "id": "acdream.example",
+      "displayName": "Example",
+      "version": "0.1.0",
+      "entryDll": "example.dll",
+      "apiVersion": 1
+    }
+    """;
+
+    [Fact]
+    public void Scan_EmptyDirectory_ReturnsEmpty()
+    {
+        var results = PluginDiscovery.Scan(_tempDir);
+        Assert.Empty(results);
+    }
+
+    [Fact]
+    public void Scan_NonexistentDirectory_ReturnsEmpty()
+    {
+        var results = PluginDiscovery.Scan(Path.Combine(_tempDir, "does-not-exist"));
+        Assert.Empty(results);
+    }
+
+    [Fact]
+    public void Scan_OneValidPlugin_ReturnsOneSuccess()
+    {
+        var pluginDir = WriteManifest("example", ValidManifest);
+
+        var results = PluginDiscovery.Scan(_tempDir);
+
+        var single = Assert.Single(results);
+        Assert.True(single.Success);
+        Assert.NotNull(single.Manifest);
+        Assert.Equal("acdream.example", single.Manifest!.Id);
+        Assert.Equal(pluginDir, single.PluginDirectory);
+    }
+
+    [Fact]
+    public void Scan_SubdirWithoutManifest_IsSkipped()
+    {
+        Directory.CreateDirectory(Path.Combine(_tempDir, "noplugin"));
+        WriteManifest("real", ValidManifest);
+
+        var results = PluginDiscovery.Scan(_tempDir);
+
+        Assert.Single(results);
+    }
+
+    [Fact]
+    public void Scan_InvalidManifest_ReturnsFailureButKeepsGoing()
+    {
+        WriteManifest("broken", "{ not json");
+        WriteManifest("ok", ValidManifest);
+
+        var results = PluginDiscovery.Scan(_tempDir).ToList();
+
+        Assert.Equal(2, results.Count);
+        Assert.Contains(results, r => !r.Success && r.Error != null);
+        Assert.Contains(results, r => r.Success);
+    }
+}