4b - the walk reports its headroom, so a list falling back to the directive form refuses at zero and the list limit stays 500

src/nesting.ts

@@ -1,2 +1,2 @@
-// A block's depth is its count of block ancestors, a list item's children two below the list, in either direction and whichever spelling holds them — or the two guards disagree.
+// Levels count per AGENTS.md §11.
 export const largestNesting = 500