chore: replace demo app with react-native library scaffold

Repo becomes the myid-sdk npm package: library at root, RN 0.86.2 vanilla
example app in example/, Turbo Module codegen enabled, yarn 4 workspace.

The previous demo app targeted MyID SDK 2.x (clientId, withPassportData,
MyIdBuildMode), an API that no longer exists in 3.x, so it is removed rather
than migrated. Recoverable via the pre-rewrite tag.

src/index.tsx

+export { multiply } from './multiply';

src/multiply.native.tsx

+import MyidSdk from './NativeMyidSdk';
+
+export function multiply(a: number, b: number): number {
+  return MyidSdk.multiply(a, b);
+}

src/multiply.tsx

+export function multiply(_a: number, _b: number): number {
+  throw new Error("'myid-sdk' is only supported on native platforms.");
+}

tsconfig.build.json

+{
+  "extends": "./tsconfig",
+  "exclude": ["example", "lib"]
+}

tsconfig.json

 {
-  "extends": "@react-native/typescript-config/tsconfig.json"
+  "compilerOptions": {
+    "rootDir": ".",
+    "paths": {
+      "myid-sdk": ["./src/index"]
+    },
+    "allowUnreachableCode": false,
+    "allowUnusedLabels": false,
+    "customConditions": [
+      "myid-sdk-source",
+      "react-native-strict-api"
+    ],
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "jsx": "react-jsx",
+    "lib": ["ESNext"],
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "noEmit": true,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitReturns": true,
+    "noUncheckedIndexedAccess": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "resolveJsonModule": true,
+    "skipLibCheck": true,
+    "strict": true,
+    "target": "ESNext",
+    "verbatimModuleSyntax": true
+  }
 }

turbo.json

+{
+  "$schema": "https://turbo.build/schema.json",
+  "globalDependencies": [".nvmrc", ".yarnrc.yml"],
+  "globalEnv": ["NODE_ENV"],
+  "tasks": {
+    "build:android": {
+      "env": ["ANDROID_HOME", "ORG_GRADLE_PROJECT_newArchEnabled"],
+      "inputs": [
+        "package.json",
+        "android",
+        "!android/build",
+        "src/*.ts",
+        "src/*.tsx",
+        "example/package.json",
+        "example/android",
+        "!example/android/.gradle",
+        "!example/android/build",
+        "!example/android/app/build"
+      ],
+      "outputs": []
+    },
+    "build:ios": {
+      "env": [
+        "RCT_NEW_ARCH_ENABLED",
+        "RCT_REMOVE_LEGACY_ARCH",
+        "RCT_USE_RN_DEP",
+        "RCT_USE_PREBUILT_RNCORE"
+      ],
+      "inputs": [
+        "package.json",
+        "*.podspec",
+        "ios",
+        "src/*.ts",
+        "src/*.tsx",
+        "example/package.json",
+        "example/ios",
+        "!example/ios/build",
+        "!example/ios/Pods"
+      ],
+      "outputs": []
+    }
+  }
+}