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)
Addresses code quality review of c082ecf:
- Require takes a literal JSON field name, no more fragile PascalCase->camelCase transform
- Parse_MissingRequiredField_Throws asserts exact message, not substring
- Remove unused using System.Text.Json.Serialization
