feat: add virtual System module for complete module listing

Add a virtual System module builder that provides System entities
(User, Session, FileDocument, Image, etc.) and associations when
listing modules and domain models. This enables catalog queries and
reference resolution against System types without requiring them
to exist in the MPR file.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: engalar

sdk/mpr/reader_documents.go

@@ -33,6 +33,9 @@ func (r *Reader) ListModules() ([]*model.Module, error) {
 		modules = append(modules, module)
 	}
 
+	// Append virtual System module
+	modules = append(modules, BuildSystemModule())
+
 	return modules, nil
 }
 
@@ -102,6 +105,9 @@ func (r *Reader) ListDomainModels() ([]*domainmodel.DomainModel, error) {
 		}
 	}
 
+	// Append virtual System module domain model
+	domainModels = append(domainModels, BuildSystemDomainModel())
+
 	return domainModels, nil
 }