feat(core): add PluginLoader with collectible ALC

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}");
+        }
+    }
+}