rtx-fsi harness: 'fixed' coupler (constant relaxation OMEGA0) for the P6-c Robin–Neumann test — at ω = 1 the candidate is the FEA's own output, so the Robin wall's previous-pass datum is the structure's traction at the candidate; design 2's FSI3 divergence is the datum's inconsistency under IQN's ω0 = 0.05

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YJPeT6WA2e7YvAnS875AHL

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

@@ -471,6 +471,15 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         let mut outcome = if let Some(iqn) = iqn.as_mut() {
             iqn.set_tolerance(tol_step).unwrap();
             iqn.solve(&d_predicted, pass)
+        } else if cfg.coupler == "fixed" {
+            // P6-c (`docs/overset_metal_campaign.md` §5.19): the constant
+            // relaxation `OMEGA0` — at 1.0 the plain staggered loop, the
+            // only one whose candidate is the FEA's own output, so the
+            // Robin wall's datum (the previous pass's tractions) IS the
+            // structure's traction at the candidate (Robin–Neumann).
+            Subiterated::relaxed(cfg.initial_relaxation, cfg.max_subiterations, tol_step)
+                .unwrap()
+                .solve(&d_predicted, pass)
         } else {
             Subiterated::aitken(cfg.max_subiterations, tol_step)
                 .unwrap()