fix(markdown): highlight YAML frontmatter in the source editor

The markdown grammar's frontMatter rule embeds source.yaml at the start
of the document, but Orca loaded YAML only when a ```yaml fence label
was seen, so frontmatter stayed plain text. Load the YAML grammar
together with markdown.

Fixes #41658

Co-authored-by: Garrett Peppers <garrett@pepperstech.net>

src/renderer/src/lib/monaco-languages/markdown-textmate-tokens.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it, vi } from 'vitest'
+import { loadNodeOniguruma } from '../syntax-highlighting/oniguruma-test-harness'
+import { createMarkdownTokensProvider } from './markdown-textmate-tokens'
+
+vi.mock('../syntax-highlighting/oniguruma', () => ({ loadOniguruma: loadNodeOniguruma }))
+
+describe('markdown tokenization', () => {
+  it('colors frontmatter as YAML on first paint, then returns to markdown', async () => {
+    const provider = await createMarkdownTokensProvider({
+      editor: { defineTheme: vi.fn() },
+      languages: { setTokensProvider: vi.fn() }
+    })
+    let state = provider.getInitialState()
+    const tokens = ['---', 'name: "Orca"', 'enabled: true # flag', '---', '# Heading'].map(
+      (line) => {
+        const result = provider.tokenize(line, state)
+        state = result.endState
+        return result.tokens
+      }
+    )
+
+    expect(tokens[1]).toContainEqual({ startIndex: 0, scopes: 'md.entity.name.tag.yaml' })
+    expect(tokens[1]).toContainEqual({ startIndex: 6, scopes: 'md.string.quoted.double.yaml' })
+    expect(tokens[2]).toContainEqual({ startIndex: 9, scopes: 'md.constant.language.boolean.yaml' })
+    expect(tokens[2]).toContainEqual({ startIndex: 14, scopes: 'md.comment.line.number-sign.yaml' })
+    expect(tokens[4]).toContainEqual({ startIndex: 2, scopes: 'md.entity.name.section.markdown' })
+  })
+})

src/renderer/src/lib/monaco-languages/markdown-textmate-tokens.ts

@@ -49,24 +49,27 @@ function fenceLanguagesByLabel(
   return byLabel
 }
 
-/** VS Code's markdown grammar with Dark+/Light+ colors; fence grammars load on first sight. */
+/** VS Code's markdown grammar with Dark+/Light+ colors; YAML loads up front for frontmatter, fence grammars on first sight. */
 export async function createMarkdownTokensProvider(
   monaco: MonacoModule
 ): Promise<Monaco.languages.TokensProvider> {
   const [
     editorCore,
     { default: markdownGrammars },
+    { default: yamlGrammars },
     { bundledLanguagesInfo },
     { rules: darkRules },
     { rules: lightRules }
   ] = await Promise.all([
     loadEditorCore(),
     import('shiki/langs/markdown.mjs'),
+    import('shiki/langs/yaml.mjs'),
     import('shiki/langs'),
     loadBuiltInMarkdownRules('dark'),
     loadBuiltInMarkdownRules('light')
   ])
-  await editorCore.core.loadLanguage(...markdownGrammars)
+  // Why: frontmatter embeds YAML at the document start, with no fence label to trigger a lazy load.
+  await editorCore.core.loadLanguage(...markdownGrammars, ...yamlGrammars)
 
   // Why: Orca's editors all use the built-in theme names; redefining them keeps every setTheme caller unchanged.
   monaco.editor.defineTheme('vs-dark', {