test(rag): make TempDir names unique across parallel tests

SystemTime nanos alone collide on coarse clocks, so concurrent tests shared
a scratch dir and read each other's generated sbx mixins (flaky
generated_sbx_mixin_* failures in CI).

src/rag/mod.rs

@@ -2289,10 +2289,17 @@ mod tests {
 
     impl TempDir {
         fn new(tag: &str) -> Self {
-            let unique = SystemTime::now()
+            // Wall-clock nanos alone are not unique across parallel tests
+            // (coarse clocks hand the same tick to concurrent callers, so two
+            // tests would share a directory and overwrite each other's files);
+            // a process-wide counter makes every name distinct.
+            static SEQ: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);
+            let nanos = SystemTime::now()
                 .duration_since(UNIX_EPOCH)
                 .unwrap()
                 .as_nanos();
+            let seq = SEQ.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+            let unique = format!("{nanos}-{seq}");
             let path = env::temp_dir().join(format!("coyote-rag-{tag}-{unique}"));
             fs::create_dir_all(&path).unwrap();
             Self { path }