Draft schema for vyr-north-star.xml. Current XML shape: - root element: vyr-north-star - one required introduction element (document-level prose) - one or more requirement elements - each requirement has required @id and plain text content This schema validates structure and basic constraints while leaving requirement vocabulary evolution to the XML source of truth. Prose introduction for document-level context, including modality terms and known uncertainty areas. Requirement body is currently prose text (no nested child elements in the canonical XML). mixed=true keeps forward compatibility if inline markup is introduced later. Lowercase single-word English identifiers (e.g. "locality", "correctness"). No hyphens or separators.