fsi2 replay: RTX_FSI2O_MAX_ROUNDS knob (3) on the prescribed-motion instrument, printed in its header — the coupled march cannot run at a tighter Schwarz cap (release dies), so the fluid side of the rounds question is measured on the replay

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01LzcjQX7tvgn87CQCyg9Cfr

crates/specialized/rtx-fsi/tests/turek_hron_fsi2_overset.rs

@@ -399,7 +399,11 @@ fn fsi2_overset_prescribed_motion() {
     );
     let t_end = t_end.min(r1);
     assert!(t0 >= r0, "replay starts at {r0}, after t0 = {t0}");
-    let mut fluid = OversetFluid::build_case(case, ny, 35, 100, 3).expect("overset fluid");
+    // `RTX_FSI2O_MAX_ROUNDS` (3): the Schwarz rounds cap — the coupled march's
+    // 3-round cap is load-bearing for the coupling's stability (every tighter
+    // cap dies at release), so the fluid side of that question is measured here.
+    let max_rounds = env_f("RTX_FSI2O_MAX_ROUNDS", 3.0) as usize;
+    let mut fluid = OversetFluid::build_case(case, ny, 35, 100, max_rounds).expect("overset fluid");
     let n = 2 * fluid.interface.wetted.len();
     assert_eq!(
         replay.d[0].len(),
@@ -413,7 +417,7 @@ fn fsi2_overset_prescribed_motion() {
     let dt = fluid.dt_fluid;
     let (d_rigid, l_rigid) = fluid.measure_force();
     println!(
-        "  PRESCRIBED ny = {ny}: dt {dt:.3e}, rigid drag {d_rigid:.2} lift {l_rigid:.2}, replay from t = {t0} (ramp {ramp_w} s) to {t_end}; patch offset {} h × {} rows, patch convection {:?}, bg convection {:?}, patch stretch {}, fillet {} m",
+        "  PRESCRIBED ny = {ny}: dt {dt:.3e}, rigid drag {d_rigid:.2} lift {l_rigid:.2}, replay from t = {t0} (ramp {ramp_w} s) to {t_end}; patch offset {} h × {} rows, patch convection {:?}, bg convection {:?}, patch stretch {}, fillet {} m, rounds cap {max_rounds}",
         std::env::var("RTX_FSI2O_PATCH_OFFSET").unwrap_or_else(|_| "6".into()),
         std::env::var("RTX_FSI2O_PATCH_ROWS").unwrap_or_else(|_| "12".into()),
         fsi2_harness::overset::patch_convection(),