refactor: Convert from C++20 to C++17 for broader compatibility

docs: Add Japanese README (README_JA.md)

Improve documentation: comprehensive SDK manual and license attribution

Implement Phase 6 and Phase 8: Tests and Documentation

Implement Phase 4-5: Complete Python bindings and examples

Implement Phase 3: OpusProvider data retrieval

Fix data/ directory build errors and add ANLZ parsing support

Stabilize GUI startup and ignore external dir

Implement full beat-link feature set

Convert codebase from C++20 to C++17 compatibility

Change C++ standard to C++17 for wider compatibility

Initial commit: Beat-Link C++ implementation