Prev/play/next (and shuffle) used to hug their ~20px glyphs in the middle
of the 62px rounded box, so most of the bubble was dead space. _box() now
takes split=True: the buttons tile the bubble interior in equal shares,
expanding to the (tightened) margins with the glyph centered in each.
Minimum button widths keep the bubbles at their old footprint — no bigger,
no smaller. Round 21 tests cover the equal split, vertical fill, edge
spans, and the preserved footprint.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
