feat(music): write an m3u path absolute when it leaves the playlist's directory

A track under the playlist's own directory still writes relative. Anything
outside it now writes the absolute path instead of a ../ chain. Playlists in
the mpd directory point into ~/music, so the relative form there was four
levels of .. and broke the moment anything moved.

I took the trade knowingly. ambience.m3u is the only playlist anywhere using
../ lines, and it reaches its audio through a sibling directory. So a newly
appended track there writes absolute while its hand-written lines stay
relative, and its header comment needs a matching edit. The alternative was
an exception for tracks inside the deployed tree, which would be a rule
serving exactly one file.

The comment I wrote for the earlier half of this argued one direction only.
Absolute survives the playlist moving, which I said. Relative survives the
library moving, which I didn't, and for a track in the same tree the ../ form
survives strictly more moves rather than fewer. Rewritten to name the trade
instead of implying there isn't one.

The boundary case is a directory whose name begins with two dots. The check
looks for a leading "../", so a real subdirectory called ..hidden sits under
the playlist and must stay relative. It has a test now, because loosening
that check to ".." would break that case and nothing else.

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