"Play this artist" awaited `GetAlbumTracks` inside a for loop — 13
sequential round trips for a 12-album artist — and every one of the
four sites doing that asked for whole track rows to read `FilePath`
off them. Five genres cost 6 MB across the IPC.
`GetFilePathsByAlbums(ids, libraryID)` and `GetFilePathsByGenres(names,
libraryID)` answer once and carry only the paths. Measured at 50 000
tracks: an artist 13 calls / 74.2 kB -> 2 / 19.2 kB, twenty albums
20 / 117.5 kB / 7.8 ms -> 1 / 26.0 kB / 1.7 ms, five genres
5 / 6 014 kB / 213 ms -> 1 / 1 291 kB / 32.6 ms, with the returned path
lists identical.
They return the paths grouped by album id or genre name rather than
flattened, because the caller owns the order — an album list is sorted
by name, not by id, and a flattened result would silently reorder a
queue — and because the album drag cache stores them per album. A
libraryID of 0 means "every library", matching an unset filter.
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