Add library entry point and package exports for consumption

Add src/index.ts barrel export re-exporting all components, types, and
utilities. Configure package.json with main, exports (root, tokens,
utils), peerDependencies for react/react-dom, and bump to 0.1.0.
Consumers can now install via local path and import directly:

  import { Button, AppShell, cn } from 'ads3-design-system'
  import 'ads3-design-system/tokens'

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

package.json

@@ -1,8 +1,14 @@
 {
   "name": "ads3-design-system",
   "private": true,
-  "version": "0.0.0",
+  "version": "0.1.0",
   "type": "module",
+  "main": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./tokens": "./src/tokens/tokens.css",
+    "./utils": "./src/lib/utils.ts"
+  },
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",
@@ -11,6 +17,10 @@
     "storybook": "storybook dev -p 6006",
     "build-storybook": "storybook build"
   },
+  "peerDependencies": {
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0"
+  },
   "dependencies": {
     "@floating-ui/react": "^0.27.19",
     "@fontsource-variable/public-sans": "^5.2.7",