* refactor(dom-css): group sources into import, export, and runtime
dom-css kept both conversion directions in one flat folder: HTML and CSS
import next to SceneGraph export, a JSX runtime next to the Tailwind JSX
printer, and CSS parsing and formatting in one file.
Sources now live in `import/`, `export/`, and `runtime/`, with the CSS
value helpers split by direction and `export/index.ts` as the `./export`
entry. Tests mirror the new folders. Public entry points are unchanged.
* docs(dom-css): add a package guide for the import, export, and runtime layout
dom-css had no AGENTS.md. The guide records the source layout, the
scene-graph-only dependency, the browser-safe `./export` entry, printed
code generation, and Base64 validation for imported documents, and the
root map links to it.
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