Summary

Toggle list items and toggle headings now export to external HTML using semantic <details>/<summary> elements and can be parsed back from them, completing the round-trip.

Closes #1936

Changes

Export (toExternalHTML)

• types.ts — Added optional childrenDOM property to toExternalHTML return type
• serializeBlocksExternalHTML.ts — Children appended to childrenDOM when set
• ToggleListItem/block.ts — Exports as <li><details><summary><p>...</p></summary></details></li>
• Heading/block.ts — Wraps in <details><summary> when isToggleable: true

Parse (parse / parseContent)

• getDetailsContent.ts — Shared helper for parsing <details>/<summary> into inline content + nested children, using ProseMirror's fitting algorithm (same pattern as getListItemContent)
• ToggleListItem/block.ts — Parses standalone <details> and <li> containing <details>
• Heading/block.ts — Parses <details> containing <summary> with <h1>-<h6>
• createSpec.ts — parseContent can now return undefined to fall through to default inline parsing
• types.ts — Updated parseContent return type to Fragment | undefined

Testing

• 7 parse test cases: toggleListItem, toggleListItemWithChildren, toggleListItemWithImage, toggleHeading, toggleHeadingWithoutChildren, standaloneDetailsSummary, mixedToggleAndBulletList
• 3 export-parse round-trip equality tests
• All 1118 tests pass

Checklist

• Code follows the project's coding standards
• Unit tests covering the new feature have been added
• All existing tests pass
• The documentation has been updated to reflect the new feature