Two endpoints rather than a box: no text, no fill, and the diagonal of
the frame with `flipV` picking which one. The three kinds share their
preset names, since the heads a file declares are what tell them apart.
ODF gives a line its own element and its heads a named marker, which
needs the draw: prefix declared in styles.xml — undeclared, the file
does not parse at all. Word keeps the frame and flips it.
Twelve presets beside the three rectangular ones. Each outline is one
polygon in utils/shapes.ts, given once: the node view draws it as an SVG
path, the ODF export scales the same points into draw:enhanced-path, and
Word gets the preset's name and draws its own geometry.
A gallery in the box toolbar and the ribbon renders its tiles from that
same table, so a new shape needs no icon. Verified through LibreOffice
from both formats; a preset we can't draw still imports as a warning.
Steffen Becker