● 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)

cpp/OnLoad.cpp

@@ -0,0 +1,21 @@
+/**
+ * OnLoad.cpp — Android JNI entry point
+ *
+ * Registers the JSI install function so Java can call it
+ * during React Native bridge initialization.
+ */
+
+#include <fbjni/fbjni.h>
+#include <jsi/jsi.h>
+#include <ReactCommon/CallInvokerHolder.h>
+
+#include "SalviumCryptoModule.h"
+
+extern "C" JNIEXPORT void JNICALL
+Java_com_salvium_crypto_ExpoSalviumCryptoModule_nativeInstall(
+    JNIEnv *env, jobject thiz, jlong jsiRuntimePtr) {
+  auto *rt = reinterpret_cast<facebook::jsi::Runtime *>(jsiRuntimePtr);
+  if (rt) {
+    salvium::install(*rt);
+  }
+}