test(core): ignore ribbon_groups microbenchmark by default

The once_lock_eliminates_allocation_after_first_call test from PR #257
is a timing microbenchmark that Box::leaks ~400k strings by design to
defeat allocator reuse. Mark it #[ignore] so it stays out of the default
suite; cache correctness is already covered by the pointer-identity test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/ocs_plugin_api/src/ribbon.rs

@@ -145,7 +145,13 @@ pub trait CadModule: Send + Sync {
 mod tests {
     use super::*;
 
+    // Timing microbenchmark, not a correctness check: it intentionally
+    // `Box::leak`s ~400k strings to defeat the allocator's reuse, so keep it out
+    // of the default suite. Run manually with `cargo test -- --ignored`.
+    // Correctness of the cache is covered by
+    // `once_lock_produces_identical_pointer_on_subsequent_calls`.
     #[test]
+    #[ignore = "microbenchmark: leaks strings by design; run with --ignored"]
     fn once_lock_eliminates_allocation_after_first_call() {
         // Helper to build a realistic module tree (2 groups, ~20 items)
         // Benchmark builds this manually for a clean before/after comparison.