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/PluginAssemblyLoadContext.cs

@@ -10,6 +10,8 @@ namespace AcDream.Core.Plugins;
 /// </summary>
 internal sealed class PluginAssemblyLoadContext : AssemblyLoadContext
 {
+    private const string AbstractionsAssemblyName = "AcDream.Plugin.Abstractions";
+
     private readonly AssemblyDependencyResolver _resolver;
 
     public PluginAssemblyLoadContext(string pluginDirectory, string pluginEntryPath)
@@ -21,7 +23,7 @@ internal sealed class PluginAssemblyLoadContext : AssemblyLoadContext
     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")
+        if (assemblyName.Name == AbstractionsAssemblyName)
             return null;
 
         var path = _resolver.ResolveAssemblyToPath(assemblyName);