* `RowAlign::Center` centers boxes, so an inline widget whose text is a different
size, or sits elsewhere in its box (a Label ink-centers its glyphs), lands off
the surrounding text's baseline, and apps had to measure fonts and fudge margins.
* A walk now carries a `Baseline` (`Auto`, `None`, `At(lpx)`): Labels report their
drawn first-row baseline, TextFlow runs their line's, and a turtle passes up its
first in-flow child's through padding and close-time shifts, like CSS inline-block.
* `finish_row_baseline` shifts each walk to the row's deepest baseline (an immovable
text row's own on the single-walk path) in the same single pass Center uses, seats
baseline-less walks on their bottom edge, and grows the row for boxes below the line.
42349d544d