Rebrand to ADS 3.0 Design System and add DESIGN.md component reference

Convert from Research Synthesiser-specific project to general-purpose ADS 3.0
design system intended to be forked for downstream applications. Add DESIGN.md
following Google Labs spec as machine-readable reference for AI coding agents.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -2,11 +2,11 @@ function App() {
   return (
     <div className="min-h-screen bg-bg text-text">
       <header className="bg-surface border-b border-border px-6 py-3">
-        <h1 className="text-lg font-semibold">SDC Design System</h1>
+        <h1 className="text-lg font-semibold">ADS 3.0 Design System</h1>
       </header>
       <main className="p-6">
         <p className="text-text-secondary">
-          Component library for the Research Synthesiser.
+          React component library implementing the ADS 3.0 design language.
         </p>
       </main>
     </div>