music: pool single-album artists into shared rows

An artist with one album got a heading and one cover on a row that fits
five, and a library is mostly single-album artists. Consecutive singles
share one grid, in place, so the page stays in artist order.

Each pooled cover keeps its artist's name above it in the same type as
a section heading. Dropping it was the first version and it was wrong:
scrolling then alternates between artists written large and small.

Measured on the probe's fixture: 4208px to 1895px, and a walk of the
page reaches all 21 covers instead of 9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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