Bump to 2.0.0 and widen SDK peer ranges to ^2.0.0

The client, core and server packages become ^2.0.0 peers and client is
now a required peer (App and AppBridge extend its Protocol class);
server stays optional for View-only consumers. Examples move to
zod ^4.2.0 to match the library peer range.

The root package is not yet published at 2.0.0, so the lockfile now
links the examples' @modelcontextprotocol/ext-apps dependency to the
repository root instead of a registry copy. This drops the stale 1.x
tarball (and its sdk 1.x peer) from the lockfile and makes
scripts/link-self.mjs a no-op.

examples/basic-host/package.json

@@ -1,7 +1,7 @@
 {
   "homepage": "https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/basic-host",
   "name": "@modelcontextprotocol/ext-apps-basic-host",
-  "version": "1.7.5",
+  "version": "2.0.0",
   "type": "module",
   "scripts": {
     "build": "tsc --noEmit && concurrently \"cross-env INPUT=index.html vite build\" \"cross-env INPUT=sandbox.html vite build\"",
@@ -11,11 +11,11 @@
     "dev": "cross-env NODE_ENV=development concurrently \"npm run watch\" \"npm run serve\""
   },
   "dependencies": {
-    "@modelcontextprotocol/ext-apps": "^1.7.0",
     "@modelcontextprotocol/client": "2.0.0",
+    "@modelcontextprotocol/ext-apps": "^2.0.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
-    "zod": "^4.1.13"
+    "zod": "^4.2.0"
   },
   "devDependencies": {
     "@types/express": "^5.0.0",