fix(ci): enable Corepack for React Native SDK (needs yarn@3.6.1)

The RN SDK declares `packageManager: "yarn@3.6.1"` in package.json, which
requires Corepack rather than the OS-installed Yarn 1.22. Previous CI run
fell through to Yarn 1.22 and failed:

  error This project's package.json defines "packageManager": "yarn@3.6.1".
  However the current global version of Yarn is 1.22.22.

Fix:
- pr-build.yml sdk_react_native: `corepack enable` before install; prefer
  `yarn install --immutable` when the project declares yarn via packageManager.
- package-sdk.sh (RN): same corepack-enable + yarn-preferred logic.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sanchitmonga22

.github/workflows/pr-build.yml

@@ -397,9 +397,18 @@ jobs:
       - uses: ./.github/actions/setup-toolchain
         with:
           platform: sdk-only
+      - name: Enable Corepack (for yarn@3.6.1 declared in packageManager)
+        run: corepack enable
       - name: Install RN workspace
         working-directory: sdk/runanywhere-react-native
-        run: npm install --legacy-peer-deps || yarn install
+        run: |
+          # The RN SDK uses yarn workspaces + yarn@3.6.1 via Corepack; fall
+          # back to npm install only as a last resort for older checkouts.
+          if [ -f yarn.lock ] || grep -q '"packageManager": "yarn@' package.json 2>/dev/null; then
+            yarn install --immutable || yarn install
+          else
+            npm install --legacy-peer-deps
+          fi
       - name: Typecheck packages
         working-directory: sdk/runanywhere-react-native
         run: |

sdk/runanywhere-react-native/scripts/package-sdk.sh

@@ -60,10 +60,18 @@ fi
 
 cd "$RN_ROOT"
 
-# Prefer yarn if a lockfile exists; fall back to npm.
+# The RN SDK declares `packageManager: "yarn@3.6.1"`, so it requires Corepack
+# (not the OS-level Yarn 1.x). Enable Corepack and use yarn; fall back to npm
+# only when yarn is genuinely unavailable.
+if grep -q '"packageManager": "yarn@' package.json 2>/dev/null; then
+    if command -v corepack >/dev/null 2>&1; then
+        corepack enable >/dev/null 2>&1 || true
+    fi
+fi
+
 if [ -f "yarn.lock" ] && command -v yarn >/dev/null 2>&1; then
     echo ">> yarn install"
-    yarn install
+    yarn install --immutable 2>/dev/null || yarn install
     HAS_YARN=1
 else
     echo ">> npm install --legacy-peer-deps"