Refuse a fence info string that decodes to the reserved carry name

AGENTS.md

@@ -151,7 +151,8 @@ One-line commit messages and PR titles; short PR summaries. No AI-attribution ma
 
 No wiki markup (§1), no network or filesystem I/O, no name→id resolution (§3), no ADF schema
 validation or exported validator — a refusal that keeps the round-trip is not schema validation,
-so the one a node carrying the same mark type twice earns stays, no shipped CSS (§4), no streaming APIs, no performance budget —
+so the one a spelled node carrying the same mark type twice earns stays, no shipped CSS (§4), no
+streaming APIs, no performance budget —
 conversions are O(n), real documents are kilobytes. A CLI is a later goal (`todo.md`), not a
 non-goal.
 

src/adf-to-markdown.test.ts

@@ -62,6 +62,7 @@ test('refuses the code block info strings the fence cannot hold', () => {
   assert.equal(code(adfToMarkdown(document({ attrs: { language: '' }, type: 'codeBlock' }))), 'ambiguous-attribute-spelling')
   assert.equal(code(adfToMarkdown(document({ attrs: { language: 'a`b' }, type: 'codeBlock' }))), 'unspellable-code-block-language')
   assert.equal(code(adfToMarkdown(document({ attrs: { language: ' sql' }, type: 'codeBlock' }))), 'unspellable-code-block-language')
+  assert.equal(code(adfToMarkdown(document({ attrs: { language: 'adf' }, type: 'codeBlock' }))), 'unspellable-code-block-language')
 })
 
 test('refuses a link destination CommonMark cannot spell', () => {

src/adf-to-markdown.ts

@@ -7,7 +7,7 @@ import { emitInlineLine } from './markdown-inline.ts'
 import { tryPipeTable } from './markdown-pipe-table.ts'
 import { carriedBlock, carryName } from './opaque-carry.ts'
 import { failure, success, type ConvertErrorPath, type Result } from './result.ts'
-import { holdsNullCharacter, isThematicBreak } from './commonmark-grammar.ts'
+import { holdsEntityReference, holdsNullCharacter, isThematicBreak } from './commonmark-grammar.ts'
 import { isAdfDocument } from './adf-document.ts'
 import { largestNesting } from './nesting.ts'
 import { fencedCodeBlock } from './backtick-runs.ts'
@@ -187,6 +187,9 @@ function spellCodeFenceInfo(language: JsonValue | undefined, path: ConvertErrorP
   if (/[`\n\r]/.test(language) || language !== language.trim()) {
     return failure('unspellable-code-block-language', 'a fence info string holds no backtick and no edge whitespace', path)
   }
+  if (holdsEntityReference(language)) {
+    return failure('unspellable-code-block-language', 'a fence info string shaped like an entity reference decodes on the way back', path)
+  }
   return success(language)
 }
 

src/markdown-inline.ts

@@ -132,6 +132,7 @@ function inlineRuns(nodes: readonly AdfNode[], depth: number, firstIndex: number
   const runs: InlineRun[] = []
   for (const [offset, node] of nodes.entries()) {
     const index = firstIndex + offset
+    // spec/flavour.md, Marks.
     const mark = carries(node) ? undefined : (node.marks ?? [])[depth]
     if (mark === undefined) {
       runs.push({ index, kind: 'plain', node })
@@ -148,7 +149,6 @@ function nodePath(context: InlineContext, index: number): ConvertErrorPath {
   return [...context.path, 'content', index]
 }
 
-// spec/flavour.md, Marks.
 function carries(node: AdfNode): boolean {
   return node.type !== 'hardBreak' && node.type !== 'text' && inlineDirective(node.type) === undefined
 }