Add comprehensive test coverage for core API classes (#38)

- AbstractObject.test.ts: Tests for inflate(), toJSON(), isReferencing(), removeReference()
- XcodeProject.test.ts: Expanded with getObject(), createModel(), getReferrers(), toJSON()
- RoundTrip.test.ts: Parse -> toJSON -> build -> parse validation for 10 fixtures
- SwiftPackage.test.ts: Tests for XCRemoteSwiftPackageReference, XCLocalSwiftPackageReference
- AbstractGroup.test.ts: Tests for mkdir(), move(), createGroup(), createFile()
- Xcode16Features.test.ts: Tests for PBXFileSystemSynchronizedRootGroup
- test-utils.ts: Shared utilities for fixtures, round-trip validation, orphan detection

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: EvanBacon