Affected: PyPI: mistune

### Summary

A ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` allows an attacker who can supply Markdown for parsing to cause denial of service. A crafted 58-byte Markdown document blocks the parser for approximately 6 seconds (measured on Apple M2, Python 3.14.3), with exponential growth per additional byte pair.

### Details

The vulnerable regex is defined in [`src/mistune/helpers.py#L20-L25`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L20-L25):

```python LINK_TITLE_RE = re.compile( r"[ \t\n]+(" r'"(?:\\' + PUNCTUATION + r'|[^"\x00])*"|' # "title" r"'(?:\\" + PUNCTUATION + r"|[^'\x00])*'" # 'title' r")" ) ```

The double-quote branch compiles to `"(?:\\[PUNCTUATION]|[^"\x00])*"`. The two alternatives inside `(A|B)*` overlap: a backslash followed by a punctuation character (e.g. `\!`) can be matched by **either** branch — as a 2-character escaped-punctuation sequence `\\!`, or as two individual `[^"\x00]` characters (`\` then `!`). The same ambiguity exists in the single-quoted title branch.

When the input contains repeated `\!` pairs with no closing `"`, the regex engine exhaustively backtracks through all 2^N combinations, resulting in **exponential O(2^N) time complexity**.

This is reachable through normal Markdown parsing via two code paths: 1. **Inline links**: `[text](url "PAYLOAD)` → [`parse_link()`](https://github.com/lepture/mistune/blob/df23edd60b43b6…