R2-b: the added-mass pins print the confined target (potential flow 1.071 in the unit box + Stokes 0.064 = 1.135); R4 probe: RTX_FSI2O_STEP_CSV — every step's raw force with the accepted fluid step's overlap defects (the march CSV carries 10-step medians)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs

@@ -376,6 +376,16 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         writeln!(f, "t,ux,uy,drag,lift").unwrap();
         f
     });
+    // R4 probe (2026-09-21): every step's RAW force (the CSV above carries
+    // 10-step medians) with the accepted fluid step's overlap defects —
+    // `RTX_FSI2O_STEP_CSV=<path>`: t, ux, uy, drag, lift, Σ|div| on the
+    // patch acceptors, Σ|div| on the background fringe, the patch's max
+    // cell imbalance.
+    let mut step_csv = std::env::var("RTX_FSI2O_STEP_CSV").ok().map(|p| {
+        let mut f = std::fs::File::create(p).expect("step csv");
+        writeln!(f, "t,ux,uy,drag,lift,defect_patch,defect_bg,patch_div").unwrap();
+        f
+    });
     let (t_fluid, t_structure) = (std::cell::Cell::new(0.0_f64), std::cell::Cell::new(0.0_f64));
     // PERF-2 P0: the FEA predictor step (outside both buckets before).
     let t_predictor = std::cell::Cell::new(0.0_f64);
@@ -560,6 +570,14 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         ux_series.push(ux);
         uy_series.push(uy);
         let (drag_now, lift_now) = fluid.borrow().measure_force();
+        if let Some(f) = step_csv.as_mut() {
+            let (dp, db, pd) = fluid.borrow().last_defects.get();
+            writeln!(
+                f,
+                "{t_now:.6},{ux:.6e},{uy:.6e},{drag_now:.6e},{lift_now:.6e},{dp:.3e},{db:.3e},{pd:.3e}"
+            )
+            .unwrap();
+        }
         interval_drag.push(drag_now);
         interval_lift.push(lift_now);
         if (step + 1) % 10 == 0 {