feat(music): radio-browser search command + keybinding (phase 2)

I wired up the interactive side of the radio-browser lookup: cj/music-radio-search, bound to S in the EMMS playlist buffer (s stays emms-stop). It searches, lists the matches annotated with codec, bitrate, country, votes, and tags, and lets you pick several one at a time. Selection is a completing-read loop with a "[done]" sentinel that removes each pick from the pool, so a station name with a comma can't be mis-split. Each pick is written into the save directory (no-URL stations skipped and named, colliding filenames disambiguated by UUID), then the selection plays through mpv, interrupting whatever was on.

The dedup and write-collect logic are unit-tested. The picker and playback are left to a manual check.

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