refactor(core): harden PluginLoader per code review

Addresses code quality review of a7f0732:
- LoadedPlugin now holds the AssemblyLoadContext explicitly so Task 10
  can call Unload() for hot reload (Critical)
- LoadedPlugin.Error is Exception? to match PluginDiscoveryResult and
  preserve stack traces; synthetic failures build FileNotFoundException
  and InvalidOperationException (Important)
- PluginLoader falls back to ReflectionTypeLoadException.Types if
  GetTypes() can't fully resolve (Important)
- Hardcoded abstractions assembly name is now a const (Minor)

src/AcDream.Core/Plugins/LoadedPlugin.cs

@@ -1,11 +1,20 @@
+using System.Runtime.Loader;
 using AcDream.Plugin.Abstractions;
 
 namespace AcDream.Core.Plugins;
 
+/// <summary>
+/// Outcome of a plugin load attempt.
+/// <para>On success, <see cref="Plugin"/> is the instantiated plugin, <see cref="LoadContext"/>
+/// owns its assembly, and <see cref="Error"/> is null.</para>
+/// <para>On failure, <see cref="Plugin"/> and <see cref="LoadContext"/> are null and
+/// <see cref="Error"/> describes what went wrong.</para>
+/// </summary>
 public sealed record LoadedPlugin(
     PluginManifest Manifest,
     IAcDreamPlugin? Plugin,
-    string? Error)
+    AssemblyLoadContext? LoadContext,
+    Exception? Error)
 {
     public bool Success => Plugin is not null && Error is null;
 }