The mental model

GET  /?q=acme&sort=PriceAscending
       Mirror.reduce ─▶ init ─▶ prepare (cookie, Data.satisfy) ─▶ render whole ─▶ HTML + envelope
       scripts on:  SSR.hydrate resumes the Model; nothing is read again

POST /?q=acme   (foldkit-plus-message = the form's encoded Message)
       Origin checked ─▶ init, prepare, as for the GET ─▶ update(Message)
         ─▶ 303 to /?q=acme, with the cookie written from the Model it reached
       scripts on:  the runtime answers the submit; nothing is posted

Six rules follow from it:

1. Everything shown is in the HTML. Read before the render, so no row waits for a script to price it.
2. The address is the state a GET carries. Search and filters are a GET form over the keys the URL mirror reads.
3. Both kinds of control are drawn on both sides. The plain ones show under (scripting: none), the scripted ones otherwise, so the browser's first render is the page the server sent. Never <noscript>.
4. A virtual list pages by links. Without scripts it draws one window, so it links to the next.
5. A change is a form whose submit is a Message. The runtime answers it when it runs; the server answers the post when it does not.
6. A test runs with JavaScript off, and another checks that the scripted page did not change.