● Restructure native module for Expo auto-discovery, fix 3 test failures

Move native code from native/ to Expo-compatible layout with android/
  and ios/ at package root. Add Android JNI bridge (OnLoad.cpp,
  ExpoSalviumCryptoModule.java, ExpoSalviumCryptoPackage.java).
  Prebuilt binaries now go in prebuilt/ instead of native/lib/.

  Test fixes:
  - wallet-sync: update stale DEFAULT_BATCH_SIZE assertion (10 -> 100)
  - bulletproofs+: fix hashToPoint -> hashToPointMonero rename in test,
    fix serializeProof to include V array matching parseProof format
  - transaction-builder: expect 2 outputs for exact-amount tx (zero-change
    output is always added for privacy)

android/CMakeLists.txt

@@ -0,0 +1,31 @@
+cmake_minimum_required(VERSION 3.18)
+project(ExpoSalviumCrypto)
+
+set(CMAKE_CXX_STANDARD 17)
+
+# React Native JSI headers (provided by React Native)
+find_package(fbjni REQUIRED CONFIG)
+find_package(ReactAndroid REQUIRED CONFIG)
+
+# Pre-built Rust .so
+add_library(salvium_crypto SHARED IMPORTED)
+set_target_properties(salvium_crypto PROPERTIES
+    IMPORTED_LOCATION "${CMAKE_SOURCE_DIR}/../prebuilt/android/${ANDROID_ABI}/libsalvium_crypto.so"
+)
+
+# JSI bridge module
+add_library(${PROJECT_NAME} SHARED
+    ../cpp/SalviumCryptoModule.cpp
+    ../cpp/OnLoad.cpp
+)
+
+target_include_directories(${PROJECT_NAME} PRIVATE
+    ../cpp
+    ../crates/salvium-crypto/include
+)
+
+target_link_libraries(${PROJECT_NAME}
+    salvium_crypto
+    ReactAndroid::jsi
+    fbjni::fbjni
+)

android/build.gradle

@@ -0,0 +1,51 @@
+// ExpoSalviumCrypto — Android build config
+// Links the Rust-compiled .so + C++ JSI module
+
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+    dependencies {
+        classpath "com.android.tools.build:gradle:8.1.0"
+    }
+}
+
+apply plugin: "com.android.library"
+
+android {
+    namespace "com.salvium.crypto"
+    compileSdkVersion 34
+
+    defaultConfig {
+        minSdkVersion 21
+        targetSdkVersion 34
+
+        externalNativeBuild {
+            cmake {
+                cppFlags "-std=c++17 -fexceptions"
+                arguments "-DANDROID_STL=c++_shared"
+            }
+        }
+
+        ndk {
+            abiFilters "arm64-v8a", "armeabi-v7a", "x86_64"
+        }
+    }
+
+    externalNativeBuild {
+        cmake {
+            path "CMakeLists.txt"
+        }
+    }
+
+    sourceSets {
+        main {
+            jniLibs.srcDirs = ["../prebuilt/android"]
+        }
+    }
+}
+
+dependencies {
+    implementation "com.facebook.react:react-android"
+}

android/src/main/AndroidManifest.xml

@@ -0,0 +1,3 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+  package="com.salvium.crypto">
+</manifest>

android/src/main/java/com/salvium/crypto/ExpoSalviumCryptoModule.java

@@ -0,0 +1,53 @@
+package com.salvium.crypto;
+
+import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.bridge.ReactContextBaseJavaModule;
+import com.facebook.react.bridge.ReactMethod;
+import com.facebook.react.module.annotations.ReactModule;
+
+/**
+ * React Native module that installs the JSI crypto bindings.
+ *
+ * The actual crypto functions are exposed via JSI (C++ -> Rust FFI),
+ * not through the React Native bridge. This Java module only handles
+ * loading the native library and calling the C++ install function.
+ */
+@ReactModule(name = ExpoSalviumCryptoModule.NAME)
+public class ExpoSalviumCryptoModule extends ReactContextBaseJavaModule {
+    public static final String NAME = "ExpoSalviumCrypto";
+
+    static {
+        System.loadLibrary("ExpoSalviumCrypto");
+        System.loadLibrary("salvium_crypto");
+    }
+
+    public ExpoSalviumCryptoModule(ReactApplicationContext reactContext) {
+        super(reactContext);
+    }
+
+    @Override
+    public String getName() {
+        return NAME;
+    }
+
+    /**
+     * Called from JS to install the JSI bindings.
+     * Must be called after the JSI runtime is available.
+     */
+    @ReactMethod(isBlockingSynchronousMethod = true)
+    public boolean install() {
+        try {
+            ReactApplicationContext context = getReactApplicationContext();
+            long jsiRuntimePtr = context.getJavaScriptContextHolder().get();
+            if (jsiRuntimePtr == 0) {
+                return false;
+            }
+            nativeInstall(jsiRuntimePtr);
+            return true;
+        } catch (Exception e) {
+            return false;
+        }
+    }
+
+    private native void nativeInstall(long jsiRuntimePtr);
+}

android/src/main/java/com/salvium/crypto/ExpoSalviumCryptoPackage.java

@@ -0,0 +1,24 @@
+package com.salvium.crypto;
+
+import com.facebook.react.ReactPackage;
+import com.facebook.react.bridge.NativeModule;
+import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.uimanager.ViewManager;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class ExpoSalviumCryptoPackage implements ReactPackage {
+    @Override
+    public List<NativeModule> createNativeModules(ReactApplicationContext reactContext) {
+        List<NativeModule> modules = new ArrayList<>();
+        modules.add(new ExpoSalviumCryptoModule(reactContext));
+        return modules;
+    }
+
+    @Override
+    public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {
+        return Collections.emptyList();
+    }
+}