fix: raise ChordFormatError when detokenize produces empty bars

A sequence of only metadata tokens followed by EOS would silently return a
ChordPeriod with bars=[], which would later crash or produce an empty .chord
file. Now raises immediately with a descriptive message. Added a failing-then-
passing test to cover this path.

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

tests/test_tokenizer.py

@@ -16,6 +16,7 @@ from src.tokenizer import (
     ID_TO_TOKEN,
     TOKEN_TO_ID,
     VOCAB,
+    ChordFormatError,
     ChordPeriod,
     detokenize_to_period,
     parse_chord_file,
@@ -206,3 +207,17 @@ class TestRoundTrip:
         t = parse_chord_symbol(recovered.bars[0][0])
         assert t.root == "A"
         assert t.quality == "m"
+
+    def test_empty_bar_sequence_raises(self):
+        # BOS + metadata only, then EOS — no body tokens → must raise
+        ids = [
+            TOKEN_TO_ID["<BOS>"],
+            TOKEN_TO_ID["MODE_major"],
+            TOKEN_TO_ID["TIME_4/4"],
+            TOKEN_TO_ID["SUB_4"],
+            TOKEN_TO_ID["STYLE_H1K0"],
+            TOKEN_TO_ID["FUNC_chorus"],
+            TOKEN_TO_ID["<EOS>"],
+        ]
+        with pytest.raises(ChordFormatError, match="no complete bars"):
+            detokenize_to_period(ids)