line anchor in multi-line label based on first line

The line anchor (`Label.LineAnchor`) only takes the first line of a label into account. Given a multi-line label, the line anchor `Centered` won't vertically center the label at the given point.

Is this desired behaviour, since it is called `Lineanchor` and not `LabelAnchor`?

Start by tracing Label.LineAnchor handling for multi-line labels, especially the Centered case. Confirm whether vertical centering uses only the first line, then determine the intended behavior from the existing label-anchor implementation or tests; done means the behavior is clarified and covered by an appropriate regression test.

Written by the indexing model from the issue text.