refactor: replace C++ native checks with Rust

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

.github/workflows/ci.yml

@@ -192,8 +192,24 @@ jobs:
       - name: Set up Gradle
         uses: gradle/actions/setup-gradle@v4
 
+      - name: Install Rust + cargo-ndk
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal
+          . "$HOME/.cargo/env"
+          rustup target add aarch64-linux-android
+          cargo install cargo-ndk --locked
+
+      - name: Set up Android NDK
+        uses: nttld/setup-ndk@v1
+        id: ndk
+        with:
+          ndk-version: r28b
+
       - name: Build APK
-        run: cd test-app && ./gradlew --no-daemon assembleDebug
+        run: |
+          . "$HOME/.cargo/env"
+          export ANDROID_NDK_HOME=${{ steps.ndk.outputs.ndk-path }}
+          cd test-app && ./gradlew --no-daemon assembleDebug
 
       - name: Upload artifact
         uses: actions/upload-artifact@v4