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_bar_count.chord

@@ -0,0 +1,7 @@
+# title: Wrong position count
+# key: C_major
+# time: 4/4
+# subdivision: 4
+# style: user
+
+| C . . . . | G . . . |

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 . . . |

tests/fixtures/valid_b_minor.chord

@@ -0,0 +1,7 @@
+# title: B minor test
+# key: B_minor
+# time: 4/4
+# subdivision: 4
+# style: user
+
+| Bm . . . | C#m7b5 . . . | D . . . | F#7 . . . |

tests/fixtures/valid_c_major.chord

@@ -0,0 +1,9 @@
+# title: C major test
+# key: C_major
+# time: 4/4
+# subdivision: 4
+# style: user
+# function: chorus
+
+| C . . . | Am7 . . . | F/A . . . | G7 . . . | // first half
+| Em7 . . . | Am7 . . . | Dm7 . . . | G7 . . . |

tests/fixtures/valid_fsharp_major.chord

@@ -0,0 +1,7 @@
+# title: F# major test
+# key: F#_major
+# time: 4/4
+# subdivision: 4
+# style: user
+
+| F#maj7 . . . | D#m7 . . . | Bmaj7 . F#/A# . | C# . . . |

tests/fixtures/valid_gsharp_minor.chord

@@ -0,0 +1,7 @@
+# title: G# minor test
+# key: G#_minor
+# time: 4/4
+# subdivision: 4
+# style: user
+
+| G#m . . . | A#maj7 . . . | Bmaj7/F# . . . | D#7 . . . |

tests/test_chord_file_parser.py

@@ -0,0 +1,272 @@
+"""Tests for parse_chord_file() and transpose_to_canonical() in src/tokenizer.py.
+
+Fixture files live in tests/fixtures/:
+  valid_c_major.chord       — already canonical (C major, 8 bars, has a // comment)
+  valid_fsharp_major.chord  — F# major with a slash chord (F#/A#)
+  valid_b_minor.chord       — B minor
+  valid_gsharp_minor.chord  — G# minor with a slash chord (Bmaj7/F#)
+  invalid_bar_count.chord   — bar 1 has 5 positions instead of 4
+  invalid_chord_symbol.chord — bar 2 contains the invalid symbol 'Xyz'
+"""
+
+from pathlib import Path
+
+import pytest
+
+from src.chord_parser import ChordTokens, parse_chord_symbol
+from src.tokenizer import ChordFormatError, ChordPeriod, parse_chord_file, transpose_to_canonical
+
+FIXTURES = Path(__file__).parent / "fixtures"
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def fixture(name: str) -> Path:
+    return FIXTURES / name
+
+
+# ---------------------------------------------------------------------------
+# parse_chord_file — valid files
+# ---------------------------------------------------------------------------
+
+
+class TestParseChordFile:
+    def test_c_major_header_fields(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        assert p.title == "C major test"
+        assert p.key == "C_major"
+        assert p.time == "4/4"
+        assert p.subdivision == 4
+        assert p.style == "user"
+        assert p.function == "chorus"
+
+    def test_c_major_bar_count(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        assert len(p.bars) == 8
+
+    def test_c_major_bar_positions(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        assert p.bars[0] == ["C", ".", ".", "."]
+        assert p.bars[2] == ["F/A", ".", ".", "."]  # slash chord preserved verbatim
+        assert p.bars[7] == ["G7", ".", ".", "."]
+
+    def test_c_major_comments_stripped(self):
+        # The first bar line ends with '// first half'; no '//' should bleed into positions.
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        for bar in p.bars:
+            assert not any("//" in pos for pos in bar)
+
+    def test_each_bar_has_expected_position_count(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        for bar in p.bars:
+            assert len(bar) == 4  # 4/4, subdivision=4
+
+    def test_fsharp_major_parses(self):
+        p = parse_chord_file(fixture("valid_fsharp_major.chord"))
+        assert p.key == "F#_major"
+        assert len(p.bars) == 4
+        assert p.bars[0][0] == "F#maj7"
+        assert p.bars[2][2] == "F#/A#"  # slash chord preserved verbatim
+
+    def test_b_minor_parses(self):
+        p = parse_chord_file(fixture("valid_b_minor.chord"))
+        assert p.key == "B_minor"
+        assert p.function == "unspecified"  # no function header field
+        assert p.bars[0][0] == "Bm"
+        assert p.bars[1][0] == "C#m7b5"
+
+    def test_gsharp_minor_parses(self):
+        p = parse_chord_file(fixture("valid_gsharp_minor.chord"))
+        assert p.key == "G#_minor"
+        assert len(p.bars) == 4
+        assert p.bars[2][0] == "Bmaj7/F#"  # slash chord
+
+    def test_hold_positions_preserved(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        # Every position after the first in each bar is '.'
+        for bar in p.bars:
+            assert bar[1] == "."
+            assert bar[2] == "."
+            assert bar[3] == "."
+
+
+# ---------------------------------------------------------------------------
+# parse_chord_file — error cases
+# ---------------------------------------------------------------------------
+
+
+class TestParseChordFileErrors:
+    def test_invalid_bar_count_raises(self):
+        with pytest.raises(ChordFormatError):
+            parse_chord_file(fixture("invalid_bar_count.chord"))
+
+    def test_invalid_bar_count_error_mentions_bar_number(self):
+        with pytest.raises(ChordFormatError) as exc_info:
+            parse_chord_file(fixture("invalid_bar_count.chord"))
+        assert "bar 1" in str(exc_info.value)
+
+    def test_invalid_bar_count_error_mentions_filename(self):
+        with pytest.raises(ChordFormatError) as exc_info:
+            parse_chord_file(fixture("invalid_bar_count.chord"))
+        assert "invalid_bar_count.chord" in str(exc_info.value)
+
+    def test_invalid_chord_symbol_raises(self):
+        with pytest.raises(ChordFormatError):
+            parse_chord_file(fixture("invalid_chord_symbol.chord"))
+
+    def test_invalid_chord_symbol_error_mentions_bar_number(self):
+        with pytest.raises(ChordFormatError) as exc_info:
+            parse_chord_file(fixture("invalid_chord_symbol.chord"))
+        assert "bar 2" in str(exc_info.value)
+
+    def test_invalid_chord_symbol_error_mentions_filename(self):
+        with pytest.raises(ChordFormatError) as exc_info:
+            parse_chord_file(fixture("invalid_chord_symbol.chord"))
+        assert "invalid_chord_symbol.chord" in str(exc_info.value)
+
+
+# ---------------------------------------------------------------------------
+# transpose_to_canonical — F# major → C major  (shift = 6)
+# ---------------------------------------------------------------------------
+
+
+class TestTransposeFsharpMajor:
+    def setup_method(self):
+        self._period = parse_chord_file(fixture("valid_fsharp_major.chord"))
+        self._t = transpose_to_canonical(self._period)
+
+    def test_key_updated_to_c_major(self):
+        assert self._t.key == "C_major"
+
+    def test_tonic_chord_becomes_c(self):
+        # F#maj7 (bar 0) → Cmaj7
+        assert self._t.bars[0][0] == "Cmaj7"
+
+    def test_second_degree_chord(self):
+        # D#m7 (bar 1) → Am7  (D#=3, 3+6=9=A)
+        assert self._t.bars[1][0] == "Am7"
+
+    def test_fourth_degree_chord(self):
+        # Bmaj7 (bar 2 pos 0) → Fmaj7  (B=11, 11+6=17→5=F)
+        assert self._t.bars[2][0] == "Fmaj7"
+
+    def test_fifth_degree_chord(self):
+        # C# (bar 3) → G  (C#=1, 1+6=7=G)
+        tokens = parse_chord_symbol(self._t.bars[3][0])
+        assert tokens.root == "G"
+        assert tokens.quality == "maj"
+
+    def test_slash_chord_root_transposed(self):
+        # F#/A# (bar 2, pos 2): root F# → C
+        tokens = parse_chord_symbol(self._t.bars[2][2])
+        assert tokens.root == "C"
+
+    def test_slash_chord_bass_transposed(self):
+        # F#/A# (bar 2, pos 2): bass A#(=10) → E (10+6=16→4=E)
+        tokens = parse_chord_symbol(self._t.bars[2][2])
+        assert tokens.bass == "E"
+
+    def test_slash_chord_full_tokens(self):
+        tokens = parse_chord_symbol(self._t.bars[2][2])
+        assert tokens == ChordTokens("C", "maj", "none", "E")
+
+    def test_hold_positions_unchanged(self):
+        # Bars 0, 1, 3 are single-chord bars: positions 1–3 must remain '.'.
+        # Bar 2 has a chord at position 2 (F#/A# → Cmaj/E) — tested separately.
+        for bar_idx in (0, 1, 3):
+            assert all(pos == "." for pos in self._t.bars[bar_idx][1:])
+
+    def test_bar_count_preserved(self):
+        assert len(self._t.bars) == len(self._period.bars)
+
+
+# ---------------------------------------------------------------------------
+# transpose_to_canonical — G# minor → A minor  (shift = 1)
+# ---------------------------------------------------------------------------
+
+
+class TestTransposeGsharpMinor:
+    def setup_method(self):
+        self._period = parse_chord_file(fixture("valid_gsharp_minor.chord"))
+        self._t = transpose_to_canonical(self._period)
+
+    def test_key_updated_to_a_minor(self):
+        assert self._t.key == "A_minor"
+
+    def test_tonic_becomes_a(self):
+        # G#m (bar 0) → Am  (G#=8, 8+1=9=A)
+        assert self._t.bars[0][0] == "Am"
+
+    def test_second_degree_chord(self):
+        # A#maj7 (bar 1) → Bmaj7  (A#=10, 10+1=11=B)
+        assert self._t.bars[1][0] == "Bmaj7"
+
+    def test_slash_chord_root_transposed(self):
+        # Bmaj7/F# (bar 2): root B(=11) → C (11+1=0=C)
+        tokens = parse_chord_symbol(self._t.bars[2][0])
+        assert tokens.root == "C"
+
+    def test_slash_chord_bass_transposed(self):
+        # Bmaj7/F# (bar 2): bass F#(=6) → G (6+1=7=G)
+        tokens = parse_chord_symbol(self._t.bars[2][0])
+        assert tokens.bass == "G"
+
+    def test_slash_chord_full_tokens(self):
+        tokens = parse_chord_symbol(self._t.bars[2][0])
+        assert tokens == ChordTokens("C", "maj7", "none", "G")
+
+    def test_fourth_bar(self):
+        # D#7 (bar 3) → E7  (D#=3, 3+1=4=E)
+        assert self._t.bars[3][0] == "E7"
+
+
+# ---------------------------------------------------------------------------
+# transpose_to_canonical — already canonical (C major)
+# ---------------------------------------------------------------------------
+
+
+class TestTransposeCMajorIdentity:
+    def test_returns_same_object(self):
+        # Fast path: shift == 0, original period returned unchanged.
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        t = transpose_to_canonical(p)
+        assert t is p
+
+    def test_key_unchanged(self):
+        p = parse_chord_file(fixture("valid_c_major.chord"))
+        assert transpose_to_canonical(p).key == "C_major"
+
+
+# ---------------------------------------------------------------------------
+# transpose_to_canonical — B minor → A minor  (shift = 10)
+# ---------------------------------------------------------------------------
+
+
+class TestTransposeBMinor:
+    def setup_method(self):
+        self._period = parse_chord_file(fixture("valid_b_minor.chord"))
+        self._t = transpose_to_canonical(self._period)
+
+    def test_key_updated_to_a_minor(self):
+        assert self._t.key == "A_minor"
+
+    def test_tonic_becomes_a(self):
+        # Bm (B=11): 11+10=21→9=A
+        assert self._t.bars[0][0] == "Am"
+
+    def test_half_diminished_chord(self):
+        # C#m7b5 (C#=1): 1+10=11=B → Bm7b5
+        assert self._t.bars[1][0] == "Bm7b5"
+
+    def test_major_chord_transposed(self):
+        # D (D=2): 2+10=12→0=C → Cmaj
+        tokens = parse_chord_symbol(self._t.bars[2][0])
+        assert tokens.root == "C"
+        assert tokens.quality == "maj"
+
+    def test_dominant_seventh_transposed(self):
+        # F#7 (F#=6): 6+10=16→4=E → E7
+        assert self._t.bars[3][0] == "E7"