P5 audit: the overset march saves the composite at every N-th committed step (RTX_FSI2O_SAVE_EVERY: background, the deformed patch's nodes, its vectors, d, ddot, time), and fsi2_overset_audit_of_saved_instants (RTX_FSI2O_AUDIT=dir) rebuilds the composite around each saved patch (build_case_with an initial mesh, so the overlap is the instant's) and prints the solver-metric chain — the CFD23 instrument on the moving patch

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_0116sg1Qz1gMv9hdcKP1XUam

crates/specialized/rtx-fsi/tests/turek_hron_fsi2_overset.rs

@@ -18,6 +18,45 @@ const REF_UX_AMP: f64 = 12.44e-3;
 const REF_DRAG_MEAN: f64 = 208.83;
 const REF_LIFT_AMP: f64 = 234.2;
 
+/// `RTX_FSI2O_AUDIT=dir`: the solver-metric chain on every saved instant
+/// under `dir` (`inst_*`, from `RTX_FSI2O_SAVE_EVERY`), no march.
+#[test]
+fn fsi2_overset_audit_of_saved_instants() {
+    let Ok(dir) = std::env::var("RTX_FSI2O_AUDIT") else {
+        return;
+    };
+    let ny = std::env::var("RTX_FSI2O_NY")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(41);
+    let flag_nx = std::env::var("RTX_FSI2O_FLAG_NX")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(35);
+    let case = case_from_env("FSI2O", FSI2);
+    let mut dirs: Vec<_> = std::fs::read_dir(&dir)
+        .expect("audit dir")
+        .filter_map(|e| e.ok().map(|e| e.path()))
+        .filter(|p| {
+            p.file_name()
+                .and_then(|n| n.to_str())
+                .is_some_and(|n| n.starts_with("inst_"))
+        })
+        .collect();
+    dirs.sort();
+    for d in dirs {
+        let (fluid, dvec, t) =
+            fsi2_harness::overset::OversetFluid::from_instant(case, ny, flag_nx, &d)
+                .expect("instant");
+        let tip: f64 = dvec.iter().map(|v| v * v).sum::<f64>().sqrt();
+        println!(
+            "  AUDIT {} t = {t:.4} |d| {tip:.3e}: {}",
+            d.file_name().unwrap().to_string_lossy(),
+            fluid.chain_line()
+        );
+    }
+}
+
 #[test]
 fn fsi2_on_the_overset() {
     let config = OversetMarchConfig::from_env(