Isomorphic hydration in Fitz: first paint on the server, then WASM adopts the DOM

TL;DR — Mark a component hydrate and the same .fitzv does two jobs: the server renders it to HTML for a fast first paint (SEO, works-without-JS), and on boot the client-WASM runtime adopts that exact DOM — restoring the serialized state from an embedded <script>, walking the existing nodes, and wiring the event listeners — instead of wiping...