Modernize DDC to Py3/TF2, integrate FFR & DDC Onset, and add Autochart tool

- Add `ffr-difficulty-model` and `ddc_onset` as git submodules.
- Migrate `dataset/` and `learn/` scripts to Python 3.
- Replace TF 0.12 models with Keras V2 models in `learn/models_v2.py`.
- Implement `autochart.py` and `infer/autochart_lib.py` for end-to-end chart generation (single/double, 5 difficulties) and rating.
- Implement training pipeline (`scripts/prepare_data.py`, `scripts/train_all.py`).
- Replace `essentia` with `librosa` for feature extraction.
- Support batch processing in `autochart.py`.
- Add `setup.py` for package installation.
- Use `simfile` for robust SM generation and extract embedded art.

Author: google-labs-jules[bot]