Astro 6.4

Astro 6.4 ships three notable additions. First, a new pluggable `markdown.processor` API lets developers swap out the default unified-based Markdown pipeline entirely, while existing remark/rehype plugins continue to work (though top-level markdown config options are now deprecated in favor of the new API). Second, a new `@astrojs/markdown-satteri` package introduces a Rust-based Markdown processor called Sätteri that is significantly faster — cutting over a minute off build times for large docs sites like Astro's own and Cloudflare's. Sätteri does not support remark/rehype plugins but has its own plugin system. Third, the `@astrojs/cloudflare` adapter now ships `cf()` helpers that simplify wiring up experimental advanced routing with Cloudflare-specific features like KV bindings, ASSETS binding, and client address handling.