Selective hydration with mochi:hydrate

Components render server-side by default and ship zero JavaScript. Add mochi:hydrate to opt a component into client-side hydration:

<Counter mochi:hydrate count={5} />
<StaticHeader />
<!-- No JS shipped -->

Props are serialized into the HTML so they’re available during hydration. Hydration is all-or-nothing per island — the entire component subtree hydrates together.