CVE-2026-81723: NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`

XMLCorpusView._read_xml_fragment() reads a corpus file in 1 KiB blocks, appending each block to a growing fragment string, then calls _VALID_XML_RE.match(fragment) on the full accumulated buffer every iteration. Because each iteration rescans the entire accumulated fragment, the total amount of work grows quadratically with input size.

Commit c9c332284 (CWE-1333) made each match() call linear. The quadratic behavior is separate: the loop calls match() once per 1 KiB block, each time on a longer buffer.

On the test system, an 8 MiB malformed XML file consumed approximately 48 CPU-seconds through the public BNCCorpusReader.words() API with no source modification. Absolute timings vary by hardware. _read_xml_fragment() imposes no limit on fragment size or iteration count.