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>

package.json

@@ -1,5 +1,5 @@
 {
-  "name": "sdc-frontend",
+  "name": "ads3-design-system",
   "private": true,
   "version": "0.0.0",
   "type": "module",