Two separate imbalances:
- Cover art was top-aligned when width-constrained (landscape, cover_size
capped by width*0.5), leaving dead space below it instead of centering
in the available height.
- The text column always started drawing at the very top and let the
source line's bottom-pin create a big empty gap for tracks with
short/sparse metadata -- looked top-heavy and unbalanced.
Text column is now built as (height, draw_fn) pairs first so the total
height is known before committing to a starting y, then centered in the
space above the reserved source-line area.