feat: add dataset, prepare_data pipeline and fix McGill converter

- src/dataset.py: ChordDataset wrapping .pt files with pad/truncate
- scripts/prepare_data.py: tokenize .chord to .pt with train/val/holdout
  split, logs token length stats and style/function distributions
- src/external_converters/mcgill_to_chord.py: rewrite parser for real
  McGill v2 format (2-column annotation, each bar in its own pipe group,
  interval bass notation e.g. /5 and /b3)
- .gitignore: exclude data/processed/train, val, holdout subdirectories
- tests: 37 new tests for ChordDataset and converter (260 total, all pass)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -35,6 +35,9 @@ checkpoints/*.ckpt
 # Processed data (reproducible from source)
 data/processed/*.pt
 data/processed/*.pkl
+data/processed/train/
+data/processed/val/
+data/processed/holdout/
 
 # External corpora (download separately; too large for git)
 data/raw_external/