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>

src/tokenizer.py

@@ -508,6 +508,12 @@ def detokenize_to_period(token_ids: list[int]) -> ChordPeriod:
             pos_in_bar, positions_per_bar,
         )
 
+    if not bars:
+        raise ChordFormatError(
+            "token sequence produced no complete bars "
+            f"(last partial bar had {pos_in_bar}/{positions_per_bar} positions)"
+        )
+
     return ChordPeriod(
         title="detokenized",
         key=key,