Release v1.13.0

Security

• (Severity: High): Harden fenced code generation in to_markdown() by choosing backtick delimiters longer than any run inside <pre> content, preventing attacker-controlled backticks from breaking out of code blocks and exposing raw HTML to downstream Markdown renderers.
• (Severity: Low): Treat text that starts at the beginning of a rendered Markdown line as text, not block syntax, by escaping line-leading headings, blockquotes, list markers, thematic breaks, setext underlines, and fenced-code delimiters from untrusted HTML content.