Convert legacy span directions into levels.

Move BiDi Level to separate module.

Prepare for extending ProtoRun.

Move ScriptCode to appropriate module.

Fix documentation: internal references.

Fix documentation: link to layoutRich.

Fix documentation: missing boxSpacing.

Describe box edges with extensible type.

Expose piecewise paragraph construction.

Consolidate tree types into one.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Implement finding box edges.

Allow extracting ResolvedBox from WithSpan.

Implement set operations on [ResolvedBox].

Allow extracting spacing from ResolvedBox.

Store direction in ResolvedBox.

Add required textDirection to TextOptions.

Link ResolvedSpan to boxes.

Use ResolvedBox when flattening trees.