projection: visit overlay/mounted subtrees during tree iteration

yamlFrontmatter() embeds the Markdown content as an overlay on the top-level
YAML-frontmatter tree.  The previous mode (IgnoreMounts | IgnoreOverlays)
skipped that overlay entirely, so ATXHeading nodes were never visited and the
Quarto (.qmd) file outline was always empty.

Dropping the mode flag lets the iterator descend into overlay and mounted
subtrees.  This is safe because every enterNode function already filters by
node name — visiting extra nodes from foreign-language mounts is a no-op.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/source-editor/utils/tree-operations/projection.ts

@@ -1,6 +1,6 @@
 import { ensureSyntaxTree } from '@codemirror/language'
 import { EditorState, Transaction } from '@codemirror/state'
-import { IterMode, SyntaxNodeRef } from '@lezer/common'
+import { SyntaxNodeRef } from '@lezer/common'
 
 const TWENTY_MS = 20
 const FIVE_HUNDRED_MS = 500
@@ -144,7 +144,11 @@ export function getUpdatedProjection<T extends ProjectionItem>(
         }
         return enterNode(state, node, items, nodeIntersectsChange)
       },
-      mode: IterMode.IgnoreMounts | IterMode.IgnoreOverlays,
+      // Do not pass IgnoreMounts or IgnoreOverlays: mixed-language parsers
+      // (e.g. yamlFrontmatter wrapping Markdown) embed the content language
+      // as an overlay, and skipping overlays would hide all heading nodes.
+      // enterNode functions filter by node name, so visiting extra nodes from
+      // mounts/overlays in other languages is harmless.
     })
     // We know the exact projection. Return it.
     return {