feat(music): add test coverage for 7 functions, refactor with extracted helpersHEADmain

Add 47 new unit tests across 7 test files covering playlist-modified-p,
assert-valid-playlist-file, playlist-tracks, create-radio-station,
ensure-playlist-buffer, after-playlist-clear, and header-text.

Extract three helpers to reduce duplication: assert-m3u-files-exist (dedupes
2 identical guards), sync-playlist-file (dedupes 3 state-sync patterns),
and select-m3u-file (reusable M3U selection with cancel). Simplify
append-track-to-playlist nesting from 6 to 4 levels. Delete unused
cj/music-keymap-prefix variable.

0 files changed, 0 insertions, 0 deletions