R4-m: RTX_FSI2O_NODE_CSV — per-node accepted load and displacement along the flag every coupled step

Each wetted node's (dx, dy, fx, fy) grouped bottom / tip / top (sidecar
<path>.nodes.csv: group, node, reference x, y, arc length) plus the
centreline uy at s = 1/4 (mean of the two straddling nodes) and 1/2.
Default off; gate on the R4-l line slice 6.0 -> 6.2 s (CPU V-cycle,
floor 7.5e-8, rtol 1e-3): main vs branch knob-off vs knob-on march CSV
and step CSV byte-identical (md5 70dcf484..., dd535386...); the node
sums equal the step CSV's fx_nodal / fy_nodal.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

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

@@ -368,6 +368,15 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         .sqrt()
     );
 
+    let mut node_csv = super::node_csv::NodeCsv::from_env(
+        &fluid.mesh,
+        &fluid.interface,
+        [a_dofs[0], a_dofs[1]],
+        |id| {
+            let d = flag.borrow().node_dofs(id);
+            [d[0], d[1]]
+        },
+    );
     let fluid = RefCell::new(fluid);
     let mut iqn = (cfg.coupler == "iqn").then(|| {
         IqnIls::new(cfg.max_subiterations, 1.0)
@@ -626,6 +635,9 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
             )
             .unwrap();
         }
+        if let Some(nc) = node_csv.as_mut() {
+            nc.write(t_now, &flag_state, &committed_nodal);
+        }
         interval_drag.push(drag_now);
         interval_lift.push(lift_now);
         if (step + 1) % 10 == 0 {