feat(core): add PluginLoader with collectible ALC

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);
+    }
+}