● 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/SalviumCryptoModule.h

@@ -0,0 +1,32 @@
+/**
+ * SalviumCryptoModule.h — JSI HostObject for salvium-crypto
+ *
+ * Installs global.__SalviumCrypto with native crypto methods backed by
+ * the Rust static library (libsalvium_crypto.a / .so) via extern "C" FFI.
+ */
+
+#ifndef SALVIUM_CRYPTO_MODULE_H
+#define SALVIUM_CRYPTO_MODULE_H
+
+#include <jsi/jsi.h>
+#include <string>
+
+namespace salvium {
+
+using namespace facebook;
+
+class SalviumCryptoHostObject : public jsi::HostObject {
+public:
+  jsi::Value get(jsi::Runtime &rt, const jsi::PropNameID &name) override;
+  std::vector<jsi::PropNameID> getPropertyNames(jsi::Runtime &rt) override;
+};
+
+/**
+ * Install global.__SalviumCrypto on the given JSI runtime.
+ * Call this from your TurboModule's install() or from AppDelegate.
+ */
+void install(jsi::Runtime &rt);
+
+} // namespace salvium
+
+#endif /* SALVIUM_CRYPTO_MODULE_H */