CVE-2026-12876: NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars

nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.