feat: add --bars arg to control output length

generate_period() now accepts n_bars=N to stop after exactly N complete
bars. bars_completed is seeded from the prefix length so --bars counts
the full output, not just the generated tail.

scripts/generate.py exposes this as --bars (default: None = model decides).

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

src/generate.py

@@ -190,6 +190,7 @@ def generate_period(
     temperature: float = 1.0,
     top_p: float = 0.9,
     max_tokens: int = 300,
+    n_bars: Optional[int] = None,
     seed: Optional[int] = None,
 ) -> ChordPeriod:
     """Generate one harmonic period autoregressively.
@@ -210,6 +211,8 @@ def generate_period(
         temperature: Sampling temperature (> 1 = more random).
         top_p:       Nucleus cutoff probability (0 < top_p <= 1).
         max_tokens:  Hard cap on generated tokens.
+        n_bars:      Stop after this many complete bars in the output.
+                     Counts bars from the prefix too. None = let the model decide.
         seed:        RNG seed for reproducibility.
 
     Returns:
@@ -245,10 +248,15 @@ def generate_period(
     positions_per_bar = _expected_positions(time, subdivision)
 
     pos_in_bar = 0
+    bars_completed = 0
     if prefix:
         encoded_prefix, n_prefix_positions = _encode_prefix(prefix, shift_to_canonical)
         ids.extend(encoded_prefix)
         pos_in_bar = n_prefix_positions % positions_per_bar
+        bars_completed = n_prefix_positions // positions_per_bar
+
+    if n_bars is not None and bars_completed >= n_bars:
+        log.warning("prefix already spans %d bars (>= requested %d)", bars_completed, n_bars)
 
     last_id = ids[-1]
     context_limit = model.max_seq_len - 1  # leave one slot so seq_len never hits max
@@ -268,6 +276,10 @@ def generate_period(
             # Advance position counter when a body position is completed
             if (_BASS_START <= token_id <= _BASS_END) or token_id in (_HOLD, _NC):
                 pos_in_bar = (pos_in_bar + 1) % positions_per_bar
+                if pos_in_bar == 0:
+                    bars_completed += 1
+                    if n_bars is not None and bars_completed >= n_bars:
+                        break
 
             if token_id == _EOS:
                 break