Component kit: plain HTML and CSS sections, no framework #410

Build the component kit the rest of this depends on: a registry of sections that render to plain HTML and CSS, no React and no bundler.

Every kit people compare us to (VengeanceUI, Magic UI, Aceternity) ships React plus Framer Motion. Ours should paste into a WordPress block, a PHP template or a static folder and still work, which is the same promise the export already makes.

First release is six sections, each one doing a job the others cannot: hero, feature grid, pricing, testimonial, contact with the signup form from #402, and footer. Six finished beats twelve that are variants of each other. The rest follow once these hold up.

Done when: each section is defined once, renders in the app and in an export with identical markup, a test holds the two in step the way exportSectionParity.test.ts does for site sections, and each one clears #418.