Add helper function for run-length encoding.
Deduplicate emptyFont in RunSpec.
Generalise text from SpanData for other tests.
Deduplicate code in SpanData.
Add Arabic examples to spanToRuns tests.
Add shaped runs to manually test pagination.
Consolidate tree types into one.
Test interaction between boxes and mixed script.
Test soft breaking on box spacing.
Test line breaking in boxes.
Add ancestor boxes to interface.
Implement finding box edges.
Make paragraph direction explicit in tests.
Store direction in ResolvedBox.
Add required textDirection to TextOptions.
Link ResolvedSpan to boxes.
Use ResolvedBox when flattening trees.
Test layout with mixed font sizes.