feat: implement src/midi_export.py — .chord → two-track MIDI

chord_file_to_midi() parses the period in the user's original key (no
transposition), accumulates held-chord segments, then writes two pretty_midi
tracks: chords with root anchored at octave 4 (MIDI 60–71 + intervals) and
bass at octave 2 (MIDI 36–47).  Extension notes are added as a fifth voice
at their standard interval above the root.  Tempo is parameterised; the CLI
wrapper (python -m src.midi_export) supports --tempo BPM.

10 tests cover: file creation, parseability, instrument count and names,
chord/bass note counts for a 4-chord C-major fixture (14 chord + 4 bass),
octave placement assertions, and tempo affecting total duration.

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

tests/test_midi_export.py

@@ -0,0 +1,115 @@
+"""Tests for chord_file_to_midi() in src/midi_export.py.
+
+Fixture: tests/fixtures/midi_export_test.chord
+  4 bars, 4/4 sub=4, C major, no function tag.
+  Chords: C Am7 F G7 (each held 4 positions = one whole bar).
+
+Expected MIDI structure:
+  instruments[0] "Chords": 3+4+3+4 = 14 notes, pitches in octave 4–5 [60, 83]
+  instruments[1] "Bass":   4 notes (one per chord event), pitches in octave 2 [36, 47]
+"""
+
+from pathlib import Path
+
+import pretty_midi
+import pytest
+
+from src.midi_export import chord_file_to_midi
+
+FIXTURES = Path(__file__).parent / "fixtures"
+TEST_CHORD = FIXTURES / "midi_export_test.chord"
+
+
+# ---------------------------------------------------------------------------
+# Shared fixture — export once, parse back, share across test methods
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(scope="module")
+def exported(tmp_path_factory: pytest.TempPathFactory) -> pretty_midi.PrettyMIDI:
+    out = tmp_path_factory.mktemp("midi") / "out.mid"
+    chord_file_to_midi(TEST_CHORD, out)
+    return pretty_midi.PrettyMIDI(str(out))
+
+
+# ---------------------------------------------------------------------------
+# Basic output
+# ---------------------------------------------------------------------------
+
+
+class TestOutputFile:
+    def test_file_is_created(self, tmp_path):
+        out = tmp_path / "out.mid"
+        chord_file_to_midi(TEST_CHORD, out)
+        assert out.exists() and out.stat().st_size > 0
+
+    def test_midi_is_parseable(self, tmp_path):
+        out = tmp_path / "out.mid"
+        chord_file_to_midi(TEST_CHORD, out)
+        pm = pretty_midi.PrettyMIDI(str(out))
+        assert pm is not None
+
+
+# ---------------------------------------------------------------------------
+# Instrument structure
+# ---------------------------------------------------------------------------
+
+
+class TestInstruments:
+    def test_two_instruments(self, exported):
+        assert len(exported.instruments) == 2
+
+    def test_first_instrument_is_chords(self, exported):
+        assert exported.instruments[0].name == "Chords"
+
+    def test_second_instrument_is_bass(self, exported):
+        assert exported.instruments[1].name == "Bass"
+
+
+# ---------------------------------------------------------------------------
+# Note counts
+# ---------------------------------------------------------------------------
+
+
+class TestNoteCounts:
+    def test_chord_track_note_count(self, exported):
+        # C(3) + Am7(4) + F(3) + G7(4) = 14
+        assert len(exported.instruments[0].notes) == 14
+
+    def test_bass_track_note_count(self, exported):
+        # one bass note per chord event = 4
+        assert len(exported.instruments[1].notes) == 4
+
+
+# ---------------------------------------------------------------------------
+# Octave placement
+# ---------------------------------------------------------------------------
+
+
+class TestOctaves:
+    def test_chord_notes_in_middle_octave(self, exported):
+        # Root anchored at C4=60; triads/7ths add at most 11 st → max B5=83.
+        for note in exported.instruments[0].notes:
+            assert 60 <= note.pitch <= 83, f"chord pitch {note.pitch} outside [60, 83]"
+
+    def test_bass_notes_in_low_octave(self, exported):
+        # Bass anchored at C2=36; highest pitch class B2=47.
+        for note in exported.instruments[1].notes:
+            assert 36 <= note.pitch <= 47, f"bass pitch {note.pitch} outside [36, 47]"
+
+
+# ---------------------------------------------------------------------------
+# Tempo
+# ---------------------------------------------------------------------------
+
+
+class TestTempo:
+    def test_higher_tempo_shortens_duration(self, tmp_path):
+        slow = tmp_path / "slow.mid"
+        fast = tmp_path / "fast.mid"
+        chord_file_to_midi(TEST_CHORD, slow, tempo=90)
+        chord_file_to_midi(TEST_CHORD, fast, tempo=180)
+        assert (
+            pretty_midi.PrettyMIDI(str(slow)).get_end_time()
+            > pretty_midi.PrettyMIDI(str(fast)).get_end_time()
+        )