feat(core): add PluginLoader with collectible ALC

AcDream.slnx

@@ -6,6 +6,7 @@
     <Project Path="src/AcDream.Plugin.Abstractions/AcDream.Plugin.Abstractions.csproj" />
   </Folder>
   <Folder Name="/tests/">
+    <Project Path="tests/AcDream.Core.Tests.Fixtures.HelloPlugin/AcDream.Core.Tests.Fixtures.HelloPlugin.csproj" />
     <Project Path="tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj" />
   </Folder>
 </Solution>

src/AcDream.Core/Plugins/LoadedPlugin.cs

@@ -0,0 +1,11 @@
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Plugins;
+
+public sealed record LoadedPlugin(
+    PluginManifest Manifest,
+    IAcDreamPlugin? Plugin,
+    string? Error)
+{
+    public bool Success => Plugin is not null && Error is null;
+}

src/AcDream.Core/Plugins/PluginAssemblyLoadContext.cs

@@ -0,0 +1,30 @@
+using System.Reflection;
+using System.Runtime.Loader;
+
+namespace AcDream.Core.Plugins;
+
+/// <summary>
+/// Collectible ALC for a single plugin. Resolves assemblies from the plugin's
+/// own directory EXCEPT for AcDream.Plugin.Abstractions, which must come from
+/// the default (host) context so type identity for IAcDreamPlugin is preserved.
+/// </summary>
+internal sealed class PluginAssemblyLoadContext : AssemblyLoadContext
+{
+    private readonly AssemblyDependencyResolver _resolver;
+
+    public PluginAssemblyLoadContext(string pluginDirectory, string pluginEntryPath)
+        : base(name: pluginDirectory, isCollectible: true)
+    {
+        _resolver = new AssemblyDependencyResolver(pluginEntryPath);
+    }
+
+    protected override Assembly? Load(AssemblyName assemblyName)
+    {
+        // Share the abstractions assembly with the host — do NOT reload it in the plugin ALC
+        if (assemblyName.Name == "AcDream.Plugin.Abstractions")
+            return null;
+
+        var path = _resolver.ResolveAssemblyToPath(assemblyName);
+        return path is null ? null : LoadFromAssemblyPath(path);
+    }
+}

src/AcDream.Core/Plugins/PluginLoader.cs

@@ -0,0 +1,34 @@
+using System.Reflection;
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Plugins;
+
+public static class PluginLoader
+{
+    public static LoadedPlugin Load(string pluginDirectory, PluginManifest manifest, IPluginHost host)
+    {
+        var dllPath = Path.Combine(pluginDirectory, manifest.EntryDll);
+        if (!File.Exists(dllPath))
+            return new LoadedPlugin(manifest, null, $"entry dll not found: {dllPath}");
+
+        try
+        {
+            var alc = new PluginAssemblyLoadContext(pluginDirectory, dllPath);
+            var asm = alc.LoadFromAssemblyPath(dllPath);
+
+            var pluginType = asm.GetTypes()
+                .FirstOrDefault(t => !t.IsAbstract && typeof(IAcDreamPlugin).IsAssignableFrom(t));
+
+            if (pluginType is null)
+                return new LoadedPlugin(manifest, null, $"no IAcDreamPlugin implementation found in {manifest.EntryDll}");
+
+            var instance = (IAcDreamPlugin)Activator.CreateInstance(pluginType)!;
+            instance.Initialize(host);
+            return new LoadedPlugin(manifest, instance, null);
+        }
+        catch (Exception ex)
+        {
+            return new LoadedPlugin(manifest, null, $"{ex.GetType().Name}: {ex.Message}");
+        }
+    }
+}

tests/AcDream.Core.Tests.Fixtures.HelloPlugin/AcDream.Core.Tests.Fixtures.HelloPlugin.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <!-- Private=false + ExcludeAssets=runtime is CRITICAL: prevents the fixture
+         from copying AcDream.Plugin.Abstractions.dll next to itself, which
+         would cause type-identity mismatch when the ALC loads the fixture
+         and the host already has the abstractions loaded. -->
+    <ProjectReference Include="..\..\src\AcDream.Plugin.Abstractions\AcDream.Plugin.Abstractions.csproj">
+      <Private>false</Private>
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </ProjectReference>
+  </ItemGroup>
+</Project>

tests/AcDream.Core.Tests.Fixtures.HelloPlugin/HelloPlugin.cs

@@ -0,0 +1,20 @@
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Tests.Fixtures.HelloPlugin;
+
+public sealed class HelloPlugin : IAcDreamPlugin
+{
+    public int InitializeCount { get; private set; }
+    public int EnableCount { get; private set; }
+    public int DisableCount { get; private set; }
+    public IPluginHost? ReceivedHost { get; private set; }
+
+    public void Initialize(IPluginHost host)
+    {
+        ReceivedHost = host;
+        InitializeCount++;
+    }
+
+    public void Enable() => EnableCount++;
+    public void Disable() => DisableCount++;
+}

tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj

@@ -22,4 +22,13 @@
     <ProjectReference Include="..\..\src\AcDream.Core\AcDream.Core.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- Build ordering only — do NOT reference at runtime. The tests locate the
+         fixture DLL by path and load it into a separate ALC. -->
+    <ProjectReference Include="..\AcDream.Core.Tests.Fixtures.HelloPlugin\AcDream.Core.Tests.Fixtures.HelloPlugin.csproj">
+      <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
+      <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties>
+    </ProjectReference>
+  </ItemGroup>
+
 </Project>

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

@@ -0,0 +1,102 @@
+using AcDream.Core.Plugins;
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Tests.Plugins;
+
+public class PluginLoaderTests
+{
+    private static string FixturePluginPath()
+    {
+        // walk up from the test bin dir to the repo root, then into the fixture's build output
+        var baseDir = AppContext.BaseDirectory;
+        var configuration = new DirectoryInfo(baseDir).Parent!.Name; // Debug / Release
+        var repoRoot = FindRepoRoot(baseDir);
+        return Path.Combine(
+            repoRoot,
+            "tests", "AcDream.Core.Tests.Fixtures.HelloPlugin", "bin", configuration, "net10.0",
+            "AcDream.Core.Tests.Fixtures.HelloPlugin.dll");
+    }
+
+    private static string FindRepoRoot(string startDir)
+    {
+        var dir = new DirectoryInfo(startDir);
+        while (dir is not null && !File.Exists(Path.Combine(dir.FullName, "AcDream.slnx")))
+            dir = dir.Parent;
+        return dir?.FullName ?? throw new InvalidOperationException("repo root not found");
+    }
+
+    private sealed class StubHost : IPluginHost
+    {
+        public IPluginLogger Log { get; } = new StubLogger();
+    }
+
+    private sealed class StubLogger : IPluginLogger
+    {
+        public void Info(string message) { }
+        public void Warn(string message) { }
+        public void Error(string message, Exception? exception = null) { }
+    }
+
+    [Fact]
+    public void Load_FixtureDll_InstantiatesPluginAndCallsInitialize()
+    {
+        var dllPath = FixturePluginPath();
+        Assert.True(File.Exists(dllPath), $"fixture dll not found: {dllPath}");
+
+        var host = new StubHost();
+        var manifest = new PluginManifest(
+            Id: "acdream.test.hello",
+            DisplayName: "Hello",
+            Version: "0.0.1",
+            EntryDll: Path.GetFileName(dllPath),
+            ApiVersion: 1,
+            Dependencies: Array.Empty<string>());
+
+        var loaded = PluginLoader.Load(
+            pluginDirectory: Path.GetDirectoryName(dllPath)!,
+            manifest: manifest,
+            host: host);
+
+        Assert.True(loaded.Success);
+        Assert.NotNull(loaded.Plugin);
+        Assert.Equal("HelloPlugin", loaded.Plugin!.GetType().Name);
+    }
+
+    [Fact]
+    public void Load_MissingDll_ReturnsFailure()
+    {
+        var host = new StubHost();
+        var manifest = new PluginManifest(
+            Id: "x",
+            DisplayName: "X",
+            Version: "0.0.1",
+            EntryDll: "nope.dll",
+            ApiVersion: 1,
+            Dependencies: Array.Empty<string>());
+
+        var loaded = PluginLoader.Load("/does/not/exist", manifest, host);
+
+        Assert.False(loaded.Success);
+        Assert.NotNull(loaded.Error);
+    }
+
+    [Fact]
+    public void Load_DllWithNoPluginImpl_ReturnsFailure()
+    {
+        // Use AcDream.Core.dll itself — it has no IAcDreamPlugin impl
+        var coreDllDir = AppContext.BaseDirectory;
+        var host = new StubHost();
+        var manifest = new PluginManifest(
+            Id: "x",
+            DisplayName: "X",
+            Version: "0.0.1",
+            EntryDll: "AcDream.Core.dll",
+            ApiVersion: 1,
+            Dependencies: Array.Empty<string>());
+
+        var loaded = PluginLoader.Load(coreDllDir, manifest, host);
+
+        Assert.False(loaded.Success);
+        Assert.Contains("IAcDreamPlugin", loaded.Error ?? "");
+    }
+}