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/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):