fix(bench): correct NAM bench skip message to reference tests/fixtures

The model loads from tests/fixtures/, not the workspace nam/ directory.

Author: OpenSauce

rustortion-core/benches/chain.rs

@@ -127,7 +127,7 @@ fn load_first_nam_model() -> Option<String> {
 
 fn bench_nam_sample_vs_block(c: &mut Criterion) {
     let Some(model_name) = load_first_nam_model() else {
-        eprintln!("skipping NAM bench: no .nam model found in workspace nam/ directory");
+        eprintln!("skipping NAM bench: no .nam model found in tests/fixtures");
         return;
     };