[mkroemer]

Builds on #8 (headless mode and controller). Until it is merged this PR shows its commits too; the formatting part is the last commit.

This lets a host draw its own formatting toolbar for a headless viewer, with one API for documents, cells and slide text:

const f = useViewerState(ctl).format
<button aria-pressed={!!f?.bold} disabled={!f} onMouseDown={(e) => e.preventDefault()}
  onClick={() => ctl.format({ op: 'toggle', mark: 'bold' })}>B</button>

ViewerState.format is a TextFormat (bold, italic, underline, strike, font, size, color, highlight, align, list) and ViewerController.format(command) takes a FormatCommand (toggle a mark, font, size, color, highlight, align, list, clear).

• DocxEditor and PptxEditor publish their formatting state to subscribers (subscribeState) beside the toolbar's onState. A subscriber hears the current state at once, so a controller attached while text is selected reports its format without waiting for the selection to change.
• src/react/edit/format.ts maps the editors' states and the active cell's CSS to TextFormat, and commands to editor calls or cell format patches.
• The format is null while viewing, and in a presentation unless text is being edited. A command a kind has no use for does nothing (lists in cells).
• Spreadsheet undo, redo and format commands through the controller run only while editing, and canUndo / canRedo follow the editing state.

Tables, pictures, tracked changes and cell borders stay with the built-in toolbars. docs/api.md has a "Formatting" section under headless mode.

test/format.test.ts covers the mappings. e2e/controller.spec.ts bolds and resizes text in a document through the controller and checks the saved file, bolds and fills a cell, and checks that an undo while viewing leaves the cell as it is. bun run typecheck, bun test and the e2e suite pass in CI.

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