feat(model): render checkbox controls as task list tokens

[tomsideguide]

Checkbox controls had no home in the model: ListItem.checked existed but no reader set it, and readers about to extract form controls (spreadsheets, docx form fields, html inputs) would each have hand-built [x].

Adds Inline::Checkbox(bool), rendered as [x] / [ ] by the Markdown renderer, and folds ListItem.checked into it: a task list item is a list item whose content starts with a checkbox.

Watch: ListItem.checked is removed from the public model.

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Summary by cubic

Centralizes checkbox modeling by adding Inline::Checkbox(bool) and removing ListItem.checked. Markdown now renders checkboxes as task-list tokens in content; list rendering no longer injects checkbox state from the item.

• Model: add Inline::Checkbox and checkbox_text(bool); plain text includes the token; inlines_are_empty treats checkboxes as non-empty.
• Rendering: Markdown outputs [x]/[ ] and inserts a separating space before following text when needed; task-list appearance comes from an inline checkbox at the start of the first paragraph; table cells render tokens; delimiter logic unchanged.
• Bindings: Node, Python, and WASM/TypeScript add Inline.kind = 'checkbox' and Inline.checked?: boolean; remove ListItem.checked from all APIs.
• Other formats: ODF byte sizing counts a checkbox as length 3; ODF and HTML list builders stop setting checked; HTML space-boundary handling treats checkboxes as non-whitespace content.
• Tests: update task-list cases; add checkbox-in-table coverage.

Bold migrations:

• Replace all uses of ListItem.checked by inserting Inline::Checkbox(true|false) as the first inline of the item's first paragraph.
• Readers extracting form controls must emit Inline::Checkbox instead of literal [x]/[ ].
• Update consumers to read Inline.kind === 'checkbox' and Inline.checked and remove any reads of ListItem.checked.

^{Written for commit 7847603. Summary will update on new commits.}

[cubic-dev-ai]

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