Add an isolated Kotlin Multiplatform foundation

.github/workflows/test-kmp.yml

@@ -0,0 +1,89 @@
+name: Test Kotlin Multiplatform foundation
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - '.github/workflows/test-kmp.yml'
+      - '.github/actions/setup-gradle/**'
+      - '.github/actions/setup-xcode/**'
+      - 'packages/react-native/ReactShared/**'
+  push:
+    branches:
+      - main
+      - '*-stable'
+    paths:
+      - '.github/workflows/test-kmp.yml'
+      - '.github/actions/setup-gradle/**'
+      - '.github/actions/setup-xcode/**'
+      - 'packages/react-native/ReactShared/**'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: kmp-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  smoke:
+    runs-on: macos-26
+    timeout-minutes: 45
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Set up JDK 17
+        uses: actions/setup-java@v5
+        with:
+          java-version: '17'
+          distribution: 'zulu'
+      - name: Set up Xcode
+        uses: ./.github/actions/setup-xcode
+        with:
+          xcode-version: '26.4.1'
+      - name: Set up Gradle
+        uses: ./.github/actions/setup-gradle
+      - name: Cache Kotlin Native toolchain
+        uses: actions/cache@v5
+        with:
+          path: ~/.konan
+          key: kmp-foundation-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('packages/react-native/ReactShared/*.gradle.kts', 'packages/react-native/ReactShared/gradle.properties', 'packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties') }}
+      - name: Test common code and build Apple frameworks
+        working-directory: packages/react-native/ReactShared
+        run: >-
+          ./gradlew -PreactNativeSharedSmoke=true
+          jvmTest iosSimulatorArm64Test
+          linkDebugFrameworkIosArm64 linkReleaseFrameworkIosArm64
+          linkDebugFrameworkIosSimulatorArm64 linkReleaseFrameworkIosSimulatorArm64
+          linkDebugFrameworkIosX64 linkReleaseFrameworkIosX64
+          --max-workers=2 --stacktrace --console=plain
+      - name: Test Objective-C consumption
+        working-directory: packages/react-native/ReactShared
+        run: ./scripts/test-apple-smoke.sh
+      - name: Verify nonempty tests and fixture isolation
+        working-directory: packages/react-native/ReactShared
+        run: |
+          ./gradlew jvmJar --max-workers=2 --console=plain
+          python3 - <<'PY'
+          from pathlib import Path
+          import xml.etree.ElementTree as ET
+          import zipfile
+          for target in ('jvmTest', 'iosSimulatorArm64Test'):
+              suites = [ET.parse(p).getroot() for p in Path('build/smoke/test-results', target).glob('TEST-*.xml')]
+              assert suites and sum(int(s.get('tests', 0)) for s in suites) > 0, target
+              assert all(not int(s.get(k, 0)) for s in suites for k in ('failures', 'errors', 'skipped')), target
+          jars = list(Path('build/libs').glob('*.jar'))
+          assert len(jars) == 1, jars
+          with zipfile.ZipFile(jars[0]) as jar:
+              assert not any('/smoke/' in name for name in jar.namelist()), 'Test fixture leaked into default output'
+          PY
+      - name: Upload smoke reports
+        if: always()
+        uses: actions/upload-artifact@v6
+        with:
+          name: kotlin-multiplatform-smoke-results
+          path: |
+            packages/react-native/ReactShared/build/smoke/reports/tests
+            packages/react-native/ReactShared/build/smoke/test-results
+            packages/react-native/ReactShared/build/smoke/apple-interop/*.log
+          if-no-files-found: warn

packages/react-native/ReactShared/.gitignore

@@ -0,0 +1,3 @@
+/build/
+/.gradle/
+/.kotlin/

packages/react-native/ReactShared/README.md

@@ -0,0 +1,43 @@
+# Kotlin Multiplatform foundation
+
+This standalone build provides a place to assess sharing library code between
+the JVM and iOS with Kotlin Multiplatform. It has no Compose or other UI dependency.
+No React Native application, Android artifact, CocoaPods target, or npm package
+consumes this module. Normal React Native builds and runtime behavior are unchanged.
+
+The build has no production sources yet. Its small arithmetic fixture is enabled
+only by `-PreactNativeSharedSmoke=true`, and its outputs go under `build/smoke`.
+The fixture checks the compiler, common tests and Objective-C integer interop;
+it is not a proposed React Native API.
+
+## Validation
+
+Use JDK 17. Apple validation also requires macOS, Xcode and an installed iOS
+simulator. The separate Gradle wrapper isolates the Kotlin/Native plugin from the
+main Android build. CI selects Xcode 26.4.1 and uses an Apple Silicon runner.
+
+From this directory:
+
+```sh
+./gradlew -PreactNativeSharedSmoke=true jvmTest iosSimulatorArm64Test \
+  linkDebugFrameworkIosArm64 linkReleaseFrameworkIosArm64 \
+  linkDebugFrameworkIosSimulatorArm64 linkReleaseFrameworkIosSimulatorArm64 \
+  linkDebugFrameworkIosX64 linkReleaseFrameworkIosX64 \
+  --max-workers=2 --console=plain
+./scripts/test-apple-smoke.sh
+```
+
+The native runner checks Debug and Release frameworks through Objective-C on the
+host simulator. Set `RCT_KMP_SIMULATOR_UDID` to select an existing simulator.
+Intel simulator frameworks are built; Intel execution and physical-device tests
+require their respective hosts and are not part of this CI job.
+
+## Adoption is separate
+
+Each use case needs its own behavior, dependency, packaging and performance
+review before any application consumes shared code. Earlier gradient experiments
+measured additional adapter latency, allocation and Kotlin/Native runtime cost;
+this foundation does not establish that those costs are acceptable. Android AAR
+embedding, Apple runtime ownership, CocoaPods, SwiftPM and prebuilt distribution
+belong to those follow-ups. No Apple consumer or framework is designated as the
+runtime owner here.

packages/react-native/ReactShared/build.gradle.kts

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinNativeTarget
+
+plugins { kotlin("multiplatform") version "2.4.20" }
+
+group = "com.facebook.react"
+
+version = "0.0.0-local"
+
+val smokeEnabled =
+    providers.gradleProperty("reactNativeSharedSmoke").map { it.toBooleanStrict() }.getOrElse(false)
+
+if (smokeEnabled) {
+  // Keep test-only exports separate from any future production artifacts.
+  layout.buildDirectory.set(layout.projectDirectory.dir("build/smoke"))
+}
+
+kotlin {
+  explicitApi()
+  jvmToolchain(17)
+  compilerOptions {
+    // Preserve compatibility with the repository's existing Kotlin consumers.
+    languageVersion.set(KotlinVersion.KOTLIN_2_2)
+    apiVersion.set(KotlinVersion.KOTLIN_2_2)
+  }
+  jvm { compilerOptions { jvmTarget.set(JvmTarget.JVM_17) } }
+  iosArm64()
+  iosSimulatorArm64()
+  iosX64()
+
+  targets.withType<KotlinNativeTarget>().configureEach {
+    binaries.framework {
+      baseName = "ReactNativeShared"
+      isStatic = true
+      binaryOption("bundleId", "com.facebook.react.shared")
+    }
+  }
+
+  sourceSets {
+    commonMain.dependencies { implementation(kotlin("stdlib", "2.2.0")) }
+    commonTest.dependencies { implementation(kotlin("test")) }
+    if (smokeEnabled) {
+      commonMain { kotlin.srcDir("tests/smoke/kotlin") }
+      commonTest { kotlin.srcDir("tests/smoke/kotlinTest") }
+    }
+  }
+}

packages/react-native/ReactShared/gradle.properties

@@ -0,0 +1,3 @@
+org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=768m -Dfile.encoding=UTF-8
+org.gradle.caching=true
+kotlin.stdlib.default.dependency=false

packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,8 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.4.1-bin.zip
+distributionSha256Sum=2ab2958f2a1e51120c326cad6f385153bb11ee93b3c216c5fccebfdfbb7ec6cb
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists