refactor: replace fixed STYLE_user with open-ended style tag system

- STYLE_user renamed to STYLE_H1K0 in VOCAB (author's personal tag)
- Style field now accepts any [A-Za-z][A-Za-z0-9_]* identifier in .chord files
- Unknown styles fall back to STYLE_other at tokenization time with a log warning
- Test fixtures updated to style: other; drop closed _VALID_STYLES frozenset
- Spec bumped to v2.1: documents open style field, fallback behaviour, and §5.7
  guide on registering a new style token

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

tests/fixtures/invalid_bar_count.chord

@@ -2,6 +2,6 @@
 # key: C_major
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | C . . . . | G . . . |

tests/fixtures/invalid_chord_symbol.chord

@@ -2,6 +2,6 @@
 # key: C_major
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | C . . . | Xyz . . . |

tests/fixtures/midi_export_test.chord

@@ -2,6 +2,6 @@
 # key: C_major
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | C . . . | Am7 . . . | F . . . | G7 . . . |

tests/fixtures/valid_b_minor.chord

@@ -2,6 +2,6 @@
 # key: B_minor
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | Bm . . . | C#m7b5 . . . | D . . . | F#7 . . . |

tests/fixtures/valid_c_major.chord

@@ -2,7 +2,7 @@
 # key: C_major
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 # function: chorus
 
 | C . . . | Am7 . . . | F/A . . . | G7 . . . | // first half

tests/fixtures/valid_fsharp_major.chord

@@ -2,6 +2,6 @@
 # key: F#_major
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | F#maj7 . . . | D#m7 . . . | Bmaj7 . F#/A# . | C# . . . |

tests/fixtures/valid_gsharp_minor.chord

@@ -2,6 +2,6 @@
 # key: G#_minor
 # time: 4/4
 # subdivision: 4
-# style: user
+# style: other
 
 | G#m . . . | A#maj7 . . . | Bmaj7/F# . . . | D#7 . . . |

tests/test_chord_file_parser.py

@@ -40,7 +40,7 @@ class TestParseChordFile:
         assert p.key == "C_major"
         assert p.time == "4/4"
         assert p.subdivision == 4
-        assert p.style == "user"
+        assert p.style == "other"
         assert p.function == "chorus"
 
     def test_c_major_bar_count(self):

tests/test_dataset.py

@@ -21,7 +21,7 @@ def _write_pt(tmp_path: Path, stem: str, n_tokens: int) -> Path:
     """Write a dummy .pt file with sequential token IDs."""
     tokens = torch.arange(n_tokens, dtype=torch.long)
     path = tmp_path / f"{stem}.pt"
-    torch.save({"tokens": tokens, "meta": {"style": "user", "function": "verse"}}, path)
+    torch.save({"tokens": tokens, "meta": {"style": "other", "function": "verse"}}, path)
     return path
 
 

tests/test_tokenizer.py

@@ -109,7 +109,7 @@ class TestTokenizeStructure:
         assert toks[1] == "MODE_major"
         assert toks[2] == "TIME_4/4"
         assert toks[3] == "SUB_4"
-        assert toks[4] == "STYLE_user"
+        assert toks[4] == "STYLE_other"  # 'unspecified' is not in VOCAB → falls back to STYLE_other
         assert toks[5] == "FUNC_chorus"
 
     def test_bar_token_count_matches_bar_count(self):