StyleRun model: array of {start, length, style} with
CharacterStyleOverride (fontWeight, italic, textDecoration, etc.).
Renderer uses ParagraphBuilder.pushStyle/pop for mixed-style text.
TextEditor delegates paragraph building to renderer.buildParagraph().
⌘B/I/U apply to selection range when text is selected, or toggle
whole-node style when no selection. Style runs adjust on
insert/delete to preserve formatting across edits.
Fig roundtrip: import characterStyleIDs + styleOverrideTable from
TextData, export back with deduped style table.
B/I/U/S toggle buttons in TypographySection.
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