feat(music): source M3U playlists from multiple directories

The MPD radio-stream playlists moved into version control at ~/.local/share/mpd/playlists/, while the 24 local-library playlists stay at ~/music/. MPD has a single playlist_directory and its load only sees that one, so the union has to happen at the Emacs layer.

I added cj/music-m3u-roots, a precedence-ordered list of directories the reading path unions (get-m3u-files, basenames, select, load). Missing directories are skipped. On a basename collision the earlier root wins, so a local playlist shadows a same-named radio one. Saving and radio-station creation still target the single cj/music-m3u-root. Radio .m3u hold stream URLs, which the track reader and the mpv subprocess already handle, so nothing else changed to play them.

The two tests that pinned the old single-directory contract now bind the list. Their "missing directory signals an error" cases became "missing directory is skipped" to match the new behavior.

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