feat: add bigram repetition penalty to generate_period

Tracks ROOT-level bigrams (prev_root → curr_root) across chord-change events.
At each FREE position, subtracts penalty * count(prev→root) from ROOT logits,
capped at 3.0 to prevent NC/HOLD flooding at extreme values.

Practical range: 0.5 (mild, breaks loops after 2 occurrences) to 1.0
(aggressive). Default 0.0 keeps backward compatibility.

Added --repetition-penalty flag to scripts/generate.py.

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

scripts/generate.py

@@ -91,6 +91,11 @@ def main() -> None:
                     help="Hard cap on generated tokens (default: 300).")
     ap.add_argument("--bars", type=int, default=None,
                     help="Stop after this many complete bars (default: let the model decide).")
+    ap.add_argument("--repetition-penalty", type=float, default=0.0,
+                    dest="repetition_penalty",
+                    help="Bigram repetition penalty: subtracts penalty * count(prev→root) "
+                         "from ROOT logits at each chord-change position. "
+                         "0.0 = disabled (default). Suggested: 0.5–1.5.")
     ap.add_argument("--seed", type=int, default=None,
                     help="Random seed for reproducibility.")
     ap.add_argument("--tempo", type=int, default=90,
@@ -140,6 +145,7 @@ def main() -> None:
         max_tokens=args.max_tokens,
         n_bars=args.bars,
         seed=args.seed,
+        repetition_penalty=args.repetition_penalty,
     )
 
     # Give generated periods a readable title