text/layout: upstream split paragraph() into Para/place_line helpers and moved the tests to
layout/tests.rs. The branch's changes are re-applied there: the resolved paragraph direction
is passed to margins_for/avail_for/align_for (RTL point text with Justify sits flush right),
first_ascent/plausible_first and ParaLine.first/style_first (box first baseline is the d
height, vertical text keeps the ascent), and the Justify All last-line cost in
optimal_breaks. The branch's seven new layout tests moved to layout/tests.rs.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
