fix(JNI): resolve JNI code safety issues in native layer

[Federico2014]

Summary

This PR fixes critical JNI code safety issues in the native C++ layer.

Changes

LibrustzcashJNIImpl.cpp (25 functions fixed)

• Replace unused <iostream> with <cstring>
• Fix null check ordering (move BEFORE function calls)
• Add proper error cleanup with JNI_ABORT on error paths
• Use modern C++ casts (reinterpret_cast, static_cast, const_cast)
• Replace NULL with nullptr
• Fix string length API (strlen vs GetStringLength)

LibsodiumJNIImpl.cpp (8 functions fixed)

• Replace unused <iostream> with <cstring> and <new>
• Use new (std::nothrow) to prevent unhandled exceptions
• Add comprehensive null pointer checks and error cleanup
• Modern C++ style casts

CMakeLists.txt

• Add macOS-specific libc++ include path for Clang
• Support architecture-specific install destinations

README.md

• Update build instructions

Testing

• All 33 tests pass

[Federico2014]

This pr mainly fix the issue: #16.