Markdown class directive syntax not supported

[PhotonQuantum]

What version of Tailwind CSS are you using?

v4.1.7

What build tool (or framework if it abstracts the build tool) are you using?

N/A

What version of Node.js are you using?

v22.14.0

What browser are you using?

N/A

What operating system are you using?

macOS

Reproduction URL

https://github.com/PhotonQuantum/tailwind-mdx-repro

pnpx @tailwindcss/cli -i global.css -o output.css

Describe your issue

Class names in markdown generic directives (e.g. :span[Some Text]{.text-gray-500}) are not extracted. This proposal is adopted by plugins like https://github.com/remarkjs/remark-directive.