fix: the rich editor was escaping every cross-reference it touched

`[[387|Metabolic acidosis]]` is our own syntax, not CommonMark's, so
Milkdown's serializer treats the brackets as literal text and escapes
them on the way out: `\[\[387|Metabolic acidosis]]`. That renders as
literal brackets and resolves to nothing.

ArticleEditor uses RichEditor for section content, so opening an article
and saving it broke every link in it — silently, one article at a time,
against the 2,150 cross-references added earlier today. Nothing in the
database is damaged yet; nobody has edited an article since the links
were made. Found by round-tripping a stem through Milkdown while
investigating whether the question stem could move to it.

restoreMarkers is deliberately narrow: it only undoes an escape that
reconstitutes a marker we actually produce — a numeric id with a label,
or a bare slug — so an author who genuinely wrote `\[` keeps it. The
round-trip is now a test, so this cannot come back unnoticed.

Frontend 301/301.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TqXevQJhxFrM7jJg82cgZN

frontend/src/components/RichEditor.jsx

@@ -40,6 +40,25 @@ function Toolbar() {
   )
 }
 
+/**
+ * Put back the cross-reference markers Milkdown escapes on the way out.
+ *
+ * `[[387|Metabolic acidosis]]` is our own syntax, not CommonMark's, so the
+ * serializer treats the brackets as literal text and escapes them:
+ * `\[\[387|Metabolic acidosis]]`. Saving an article through the editor
+ * therefore broke every link in it — silently, one article at a time.
+ *
+ * Deliberately narrow: it only unescapes a run that reconstitutes a marker we
+ * actually produce — a numeric id with a label, or a bare slug — so an author
+ * who genuinely wrote `\[` keeps it.
+ */
+const ESCAPED_MARKER = /\\\[\\\[(\d+\|[^\]]+|[a-z0-9][a-z0-9-]*)\]\]/g
+
+export function restoreMarkers(markdown) {
+  return (markdown || '').replace(ESCAPED_MARKER, (_match, inner) => `[[${inner}]]`)
+}
+
+
 function MilkdownEditor({ value, onChange, placeholder }) {
   const isInternalChange = useRef(false)
   const lastValue = useRef(value)
@@ -52,9 +71,10 @@ function MilkdownEditor({ value, onChange, placeholder }) {
         ctx.set(defaultValueCtx, value || '')
         const l = ctx.get(listenerCtx)
         l.markdownUpdated((_, md) => {
+          const restored = restoreMarkers(md)
           isInternalChange.current = true
-          lastValue.current = md
-          onChange(md)
+          lastValue.current = restored
+          onChange(restored)
         })
       })
       .use(commonmark)

frontend/src/components/RichEditor.test.jsx

@@ -0,0 +1,61 @@
+import { describe, expect, it } from 'vitest'
+import { Editor, rootCtx, defaultValueCtx, editorViewCtx, serializerCtx } from '@milkdown/core'
+import { commonmark } from '@milkdown/preset-commonmark'
+import { gfm } from '@milkdown/preset-gfm'
+import { restoreMarkers } from './RichEditor'
+
+/** What the editor hands back for a given input, markers restored. */
+async function throughEditor(markdown) {
+  const editor = await Editor.make()
+    .config(ctx => {
+      ctx.set(rootCtx, document.createElement('div'))
+      ctx.set(defaultValueCtx, markdown)
+    })
+    .use(commonmark).use(gfm).create()
+  let out = ''
+  editor.action(ctx => { out = ctx.get(serializerCtx)(ctx.get(editorViewCtx).state.doc) })
+  await editor.destroy()
+  // The serializer adds a trailing newline; that is not what is under test.
+  return restoreMarkers(out).trim()
+}
+
+describe('restoreMarkers', () => {
+  it('puts back a cross-reference the serializer escaped', () => {
+    expect(restoreMarkers('See \\[\\[387|metabolic acidosis]] here.'))
+      .toBe('See [[387|metabolic acidosis]] here.')
+  })
+
+  it('handles the bare-slug form and several in one line', () => {
+    expect(restoreMarkers('\\[\\[croup]] and \\[\\[12|Asthma]]')).toBe('[[croup]] and [[12|Asthma]]')
+  })
+
+  it('leaves an unescaped marker alone', () => {
+    expect(restoreMarkers('plain [[7|meningitis]]')).toBe('plain [[7|meningitis]]')
+  })
+
+  it('leaves a bracket the author actually escaped', () => {
+    // Narrow on purpose: only a run that reconstitutes a real marker is undone.
+    expect(restoreMarkers('a literal \\[bracket] stays')).toBe('a literal \\[bracket] stays')
+    expect(restoreMarkers('\\[\\[not a marker!]]')).toBe('\\[\\[not a marker!]]')
+  })
+
+  it('copes with nothing', () => {
+    expect(restoreMarkers('')).toBe('')
+    expect(restoreMarkers(undefined)).toBe('')
+  })
+})
+
+describe('editing an article section does not break its cross-references', () => {
+  it('round-trips a marker through Milkdown unchanged', async () => {
+    // Without restoreMarkers this comes back as \[\[387|metabolic acidosis]],
+    // which renders as literal brackets and resolves to nothing — so opening
+    // and saving an article silently broke every link in it.
+    const stem = 'See [[387|metabolic acidosis]] for the workup.'
+    expect(await throughEditor(stem)).toBe(stem)
+  })
+
+  it('round-trips several markers in a paragraph', async () => {
+    const text = 'Consider [[12|Asthma]] and [[7|meningitis]] before [[croup]].'
+    expect(await throughEditor(text)).toBe(text)
+  })
+})