P5-1 instruments: the overset fluid saves/loads its rigid-phase state (RTX_FSI2O_SAVE / _LOAD — a coupling experiment then costs seconds, not the 12-minute rigid march), dumps the interface edges when the composite refuses a patch (RTX_FSI2O_DUMP_DIR, at the advance where the overlap is rebuilt), the release time is the fluid's clock; patch_cylinder_flag_deformed: cells inside the flag are holes at small bends and both sweep counts (the cantilever shape does not reproduce the P5-1 death — the CSVs show a coupling runaway), and a dumped-edges reproduction test

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

@@ -98,12 +98,27 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
     let dt = dt_fluid * cfg.subcycle as f64;
     let zero_d = vec![0.0; 2 * fluid.interface.wetted.len()];
 
-    // Phase 1: rigid flag to t_release.
+    // Phase 1: rigid flag to t_release (`RTX_FSI2O_LOAD=dir` replaces the
+    // march with the saved state; `RTX_FSI2O_SAVE=dir` saves it).
     let start = std::time::Instant::now();
     let rigid_steps = (cfg.t_release / dt_fluid).round() as usize;
-    for _ in 0..rigid_steps {
-        fluid.step().expect("rigid fluid step");
+    if let Ok(dir) = std::env::var("RTX_FSI2O_LOAD") {
+        let t = fluid.load(&dir).expect("load");
+        assert!(
+            (t - cfg.t_release).abs() < dt_fluid,
+            "the saved state is at t = {t}, not t_release = {}",
+            cfg.t_release
+        );
+    } else {
+        for _ in 0..rigid_steps {
+            fluid.step().expect("rigid fluid step");
+        }
+        if let Ok(dir) = std::env::var("RTX_FSI2O_SAVE") {
+            fluid.save(&dir).expect("save");
+        }
     }
+    // The release time is the fluid's clock (the rigid step count rounds).
+    let t_release = fluid.time();
     let (rigid_drag, rigid_lift) = fluid.measure_force();
     println!(
         "  {} OVERSET rigid phase: {rigid_steps} steps (dt {dt_fluid:.3e}) to t = {:.2} s in {:.0} s wall; wall drag {rigid_drag:.2} (rigid-flag reference {:.1}; the overset's own CFD2 at ny = 41: 137.8), lift {rigid_lift:.2}; rounds mean {:.2}",
@@ -280,7 +295,7 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
                 outcome = iqn_ref.solve(&d_predicted, pass);
             }
         }
-        let t_now = cfg.t_release + (step + 1) as f64 * dt;
+        let t_now = t_release + (step + 1) as f64 * dt;
         match outcome {
             Ok(c) => {
                 total_subiterations += c.iterations;