feat: implement chord symbol parser with full test suite

Adds src/chord_parser.py with parse_chord_symbol() → ChordTokens.
Handles all 18 qualities (including Unicode °/Δ/ø variants and
alternative spellings), shorthand expansion (maj9 → maj7+ext9,
C9 → 7+ext9, etc.), slash chords, and flat→sharp root normalization.
Raises ChordParseError with a descriptive message on bad input.

Adds tests/test_chord_parser.py: 90 tests covering all qualities,
all 7 extension values (including shorthands), slash chords, root
normalization, all §4.6 spec examples, and 10 invalid-input cases.

Adds requirements.txt with project dependencies.

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

requirements.txt

@@ -0,0 +1,7 @@
+torch
+music21
+pretty_midi
+pytest
+matplotlib
+numpy
+pandas