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)

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

@@ -97,6 +97,6 @@ public class PluginLoaderTests
         var loaded = PluginLoader.Load(coreDllDir, manifest, host);
 
         Assert.False(loaded.Success);
-        Assert.Contains("IAcDreamPlugin", loaded.Error ?? "");
+        Assert.Contains("IAcDreamPlugin", loaded.Error!.Message);
     }
 }