feat: implement McGill Billboard converter (Harte → .chord)

Adds src/external_converters/mcgill_to_chord.py with two public functions:
  - convert_song(song_dir, output_dir) — converts one salami_chords.txt to
    per-section .chord files (4–16 bars each, style=other)
  - convert_dataset(dataset_dir, output_dir) — batch converts all songs

Key decisions:
  - Harte qualities mapped to our 18-quality vocabulary; hdim7 → m7b5,
    parenthetical alterations (e.g. 7(b9)) handled via regex
  - Bar duration estimated from median non-trivial chord duration
  - Mode (major/minor) inferred from tonic chord quality distribution
  - Sections with <4 or >16 bars are skipped with a logged reason
  - Unrecognized Harte chords skip the whole section (no silent corruption)

48 new tests in tests/test_mcgill_converter.py; total suite 223 passed.

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

tests/fixtures/mcgill_test/0001/salami_chords.txt

@@ -0,0 +1,15 @@
+# artist: Test Artist
+# title: Test Song
+# metre: 4/4
+# tonic: C
+
+0.000000	Z
+4.000000	A,verse	C:maj
+8.000000	.	F:maj
+12.000000	.	G:7
+16.000000	.	C:maj
+20.000000	B,chorus	F:maj
+24.000000	.	C:maj
+28.000000	.	G:7
+32.000000	.	C:maj
+36.000000	Z