Test-add for the jboolean return. JNI `jboolean` is a single byte (0
or 1); sx's `bool` lowers to LLVM `i1` with byte-coercion at the ABI
boundary. CallBooleanMethod is at vtable slot 37.
IR snapshot captures today's `ret i1 undef`. Next commit adds the
`.bool => Jni.CallBooleanMethod` arm.
