Structure and Directory Reorganization

[rawbytedev]

Overview: Reorganize project files and test structure for clarity and future scalability.

Tasks

• Move to proposed hierarchical directory layout (core/, implementations/, serializers/, internal/, testing/integration/, etc.)
• Move integration tests to testing/integration/; keep implementation-specific tests close to the code
• Add internal/ for non-exported utilities
• Consider cmd/ for CLI tools
• Move test_setups.go to testing/fixtures/