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_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):