feat: implement .chord file parser and canonical transposer; freeze requirements

src/tokenizer.py:
  - parse_chord_file(Path) → ChordPeriod: reads header + bar body, strips //
    comments, validates bar position counts and chord symbols, raises
    ChordFormatError with filename and bar number on any violation.
  - transpose_to_canonical(ChordPeriod) → ChordPeriod: shifts all chord roots
    and bass notes by the semitone offset to C major / A minor; fast-path
    returns the original object when shift == 0.

tests/test_chord_file_parser.py: 39 tests covering parsing of 4 valid fixtures
  (C major, F# major, B minor, G# minor), error messages for 2 invalid
  fixtures, and transposition correctness including slash chord root+bass.

tests/fixtures/: 6 .chord fixture files (4 valid, 2 invalid).

requirements.txt: pinned to current latest stable versions
  (torch 2.12.0, music21 10.1.0, pretty_midi 0.2.11, matplotlib 3.10.9,
  numpy 2.4.6, pandas 3.0.3, pytest 9.0.3); Python >= 3.11 noted.

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

tests/fixtures/invalid_chord_symbol.chord

@@ -0,0 +1,7 @@
+# title: Invalid chord symbol
+# key: C_major
+# time: 4/4
+# subdivision: 4
+# style: user
+
+| C . . . | Xyz . . . |