The renderers draw a line from its frame's top left to its bottom right
(DrawingML's `line`; flips turn the frame), but the arrowhead code and
`end_direction` assumed a line from the bottom left to the top right, so
a sloped line's arrowheads were drawn at the other diagonal's corners,
apart from the line. Horizontal and vertical lines hid it. The
arrowheads now start from the line's own ends, pointing along it.
Regression test: `tests::a_lines_arrowheads_sit_at_its_ends`.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBVQLyDnrG8Tn1yDQcUabC