A denial of service issue exists in the `UniqueSlugNormalizer::normalize()` function. The problem occurs when the function restarts its numeric-suffix search from 1 during every slug collision, leading to O(K^2) time complexity for K headings that collapse to the same base slug. This path is triggered when the `HeadingPermalinkExtension`, `FootnoteExtension`, or `TableOfContentsExtension` is registered. An unauthenticated attacker can provide a small Markdown document containing many headings that resolve to a single base slug, such as identical heading text, punctuation-only headings, or empty ATX headings, resulting in excessive CPU consumption.