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/fsi2_harness/overset_march.rs

@@ -235,6 +235,10 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
     });
     let (t_fluid, t_structure) = (std::cell::Cell::new(0.0_f64), std::cell::Cell::new(0.0_f64));
     let prev_area = std::cell::Cell::new(fluid.borrow().shared.read().unwrap().area());
+    let save_every: usize = std::env::var("RTX_FSI2O_SAVE_EVERY")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(0);
     let phase_start = std::time::Instant::now();
 
     for step in 0..coupled_steps {
@@ -364,6 +368,18 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         committed_nodal = nodal;
         prev_area.set(fluid.borrow().shared.read().unwrap().area());
         fluid.borrow_mut().commit_base();
+        // `RTX_FSI2O_SAVE_EVERY=N` (+ `RTX_FSI2O_SAVE`): the composite at
+        // every N-th committed step, for the offline chain audit.
+        if save_every > 0 && (step + 1) % save_every == 0 {
+            if let Ok(dir) = std::env::var("RTX_FSI2O_SAVE") {
+                let d_now = extract(&flag_state);
+                let dd_now = extract_velocity(&flag_state);
+                fluid
+                    .borrow()
+                    .save_instant(&dir, step + 1, &d_now, &dd_now)
+                    .expect("save instant");
+            }
+        }
         worst_conservation = worst_conservation.max(conservation);
         faces_used = faces;
         let ux = flag_state.displacement[a_dofs[0]];