15 - refuse the directive link spelling no href, so the mark has one spelling

src/markdown/mark-spellings.ts

@@ -35,6 +35,11 @@ export function markSpelling(type: string): MarkSpelling | undefined {
   return { attributes, kind: spelling.kind }
 }
 
+export function linkHref(attrs: AdfAttributes): string | undefined {
+  const href = attrs['href']
+  return typeof href === 'string' ? href : undefined
+}
+
 // spec/flavour.md, Marks. `marksInside` counts the marks the link's nodes carry within it: the emitter's depth + 1, the parser's 0.
 export function commonMarkLink(attrs: AdfAttributes, href: string, nodes: readonly AdfNode[], marksInside: number, bracketed: boolean): CommonMarkLink | undefined {
   if (Object.keys(attrs).some((key) => key !== 'href' && key !== 'title')) return undefined