Adds VOCAB (81 tokens), TOKEN_TO_ID, and ID_TO_TOKEN per spec §5.2.
tokenize_period() transposes to C/Am then emits BOS + metadata tokens +
per-bar chord/HOLD/NC tokens + BAR + EOS. detokenize_to_period() is the
exact inverse, returning a ChordPeriod in canonical key. The m(add9)
quality maps to QUAL_m_add9 in the vocab (parentheses not valid in token
names) via _qual_token/_token_qual helpers.
36 new tests cover vocabulary integrity, token sequence structure,
and full round-trip fidelity for all four valid fixture files.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
