Parsed copy of the body for heading and fragment (hash) resolution.
For `output_mode` next_section, always filled with Markdown.Document.Render.parse on `content` (section body is markdown).
For a fenced body, filled only when the fence `lang` is exactly `markdown` or `md` (case-insensitive after strip); otherwise empty.
Omitted from the write_file tool JSON via serialize_property.