build(native-certgen): wire Rust crate into Gradle pipeline

cargo-ndk builds libcertgen.so for arm64-v8a during prepareModuleFiles.
AGP mergeJniLibFolders picks up jniLibs/ and routes through
stripped_native_libs into the module ZIP. customize.sh extracts the .so
on device install. ProGuard keeps NativeCertGen JNI class and
CertGenConfig fields for runtime JNI field access.

app/proguard-rules.pro

@@ -7,3 +7,9 @@
 -keepclasseswithmembers class org.matrix.TEESimulator.App {
     public static void main(java.lang.String[]);
 }
+
+-keepclasseswithmembers class org.matrix.TEESimulator.pki.NativeCertGen {
+    native <methods>;
+    *;
+}
+-keep class org.matrix.TEESimulator.pki.CertGenConfig { *; }