Port all 15 native VPN detection checks from C++ to Rust using
jni + libc crates. Gradle triggers cargo-ndk automatically via
a preBuild dependency — single `./gradlew installDebug` builds
everything.
- Remove CMakeLists.txt and native-lib.cpp
- Add test-app/native/ Rust crate with Cargo.toml and src/lib.rs
- Gradle buildRustNative task runs cargo-ndk, copies .so to jniLibs
- Update CI test-app job with Rust + NDK setup
- 23/23 checks pass on device
