feat: add run_pretrain.py; fix output-path naming and max_seq_len

- scripts/run_pretrain.py: single-command pre-training runner with
  timing estimate, loss-curve plot (matplotlib), and per-epoch report.
  Sets max_seq_len=256 (McGill sequences max out at 195 tokens, ~4x
  faster attention than the 512 default).
- src/train.py: normalise --output so pretrained.pt and pretrained both
  produce pretrained.pt + pretrained.log.csv (not pretrained.pt.log.csv).
  Serialize Path fields as strings in checkpoint to satisfy weights_only.
- requirements.txt: drop unused pandas/music21, add mido (pretty_midi dep).

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

src/train.py

@@ -267,8 +267,10 @@ def train(cfg: TrainConfig) -> Path:
     # ------------------------------------------------------------------
     output_path = Path(cfg.output)
     output_path.parent.mkdir(parents=True, exist_ok=True)
-    checkpoint_path = output_path.with_suffix(".pt")
-    log_csv_path = Path(str(output_path) + ".log.csv")
+    # Normalise: --output may be given with or without the .pt extension.
+    stem = output_path.with_suffix("") if output_path.suffix == ".pt" else output_path
+    checkpoint_path = stem.with_suffix(".pt")
+    log_csv_path = Path(str(stem) + ".log.csv")
 
     with open(log_csv_path, "w", newline="") as fh:
         writer = csv.writer(fh)