12c: one block opener speller beside its reader, the line readers split along the union, and AGENTS.md records the leaf body

src/markdown/emit/block-directive-spelling.ts

@@ -1,28 +1,27 @@
 import type { AdfNode } from '../../adf/document.ts'
 import type { BlockDirective } from '../../adf/block-directives.ts'
 import { blockArgument } from '../block-directive-arguments.ts'
-import { directivePrefix, isBareToken, spellAttributes, spellJsonAttribute, spellVocabulary } from '../directive-syntax.ts'
+import { isBareToken, spellAttributes, spellDirectiveOpener, spellJsonAttribute, spellVocabulary } from '../directive-syntax.ts'
 import { markValues, marksAttribute } from '../block-directive-marks.ts'
 import { nodeAttrs, nodeMarks } from '../../adf/document.ts'
 import { vocabularyPairs } from '../../adf/attribute-vocabulary.ts'
 
 export function spellBlockDirectiveOpener(node: AdfNode, directive: BlockDirective, spelledByBody: readonly string[] = []): string | undefined {
   const argumentAttribute = blockArgument(node.type)
-  const argument = spellArgument(node, argumentAttribute)
-  if (argument === undefined) return undefined
+  const slot = bareArgument(node, argumentAttribute)
+  if (slot === undefined) return undefined
   const spelled = argumentAttribute === undefined ? spelledByBody : [argumentAttribute, ...spelledByBody]
   const pairs = vocabularyPairs(nodeAttrs(node), directive.attributes, spelled)
   if (pairs === undefined) return undefined
   const spelledPairs = spellVocabulary(pairs)
   const marks = nodeMarks(node)
   if (marks.length > 0) spelledPairs.push([marksAttribute, spellJsonAttribute(markValues(marks))])
-  const attributes = spellAttributes(spelledPairs)
-  return `${directivePrefix}${node.type}${argument}${attributes === '' ? '' : ` ${attributes}`}`
+  return spellDirectiveOpener(node.type, slot.argument, spellAttributes(spelledPairs))
 }
 
-function spellArgument(node: AdfNode, argumentAttribute: string | undefined): string | undefined {
+// `undefined` where the argument slot holds a value no bare token spells.
+function bareArgument(node: AdfNode, argumentAttribute: string | undefined): { argument: string | undefined } | undefined {
   const value = argumentAttribute === undefined ? undefined : nodeAttrs(node)[argumentAttribute]
-  if (value === undefined) return ''
-  if (typeof value !== 'string' || !isBareToken(value)) return undefined
-  return ` ${value}`
+  if (value === undefined) return { argument: undefined }
+  return typeof value === 'string' && isBareToken(value) ? { argument: value } : undefined
 }