smlib builds its tests clean without the C++ kernel

The solidcore half of ssi_corpus runs without `native` (it needs no C++), and on_cpp
is unused there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MNXLXkZLQ6T7TSi4wswcmk

crates/smlib/src/route.rs

@@ -195,6 +195,7 @@ pub(crate) fn take_reached() -> (u64, u64) {
 /// guard fails a test that never touched a C++ body, so a parity test can't quietly compare
 /// solidcore with itself.
 #[cfg(test)]
+#[cfg_attr(not(feature = "native"), allow(dead_code))]
 pub(crate) fn on_cpp() -> impl Drop {
     struct Guard;
     impl Drop for Guard {

crates/smlib/tests/ssi_corpus.rs

@@ -367,7 +367,6 @@ fn parity_with_python_scorer() {
 
 /// The cases SMLib's GlobalSurfaceIntersect passes at approximation tolerance 1e-6, as score.py
 /// grades the corpus runner's output. A fix that makes another case pass adds it here.
-#[cfg(feature = "native")]
 const GSI_1E6_PASS: &[&str] = &[
     "c1_cone_plane_ellipse",
     "c1_cone_plane_hyperbola",
@@ -678,7 +677,6 @@ const SOLIDCORE_GSI_1E6_MISSES: [&str; 6] = [
 ];
 
 /// solidcore passes what the C++ kernel passes, but for [`SOLIDCORE_GSI_1E6_MISSES`].
-#[cfg(feature = "native")]
 #[test]
 fn solidcore_global_intersect_at_1e_6() {
     let Some(scores) = global_intersect_scores(solidcraft_smlib::route::Backend::Solidcore) else { return };