rtx-cfd: overset Schwarz stall rule guarded (fires only within 10× the tolerance) — unguarded it cut CFD1's transient at round 3 every step and the coupled march diverged at ny=62 (pressure 4e4 → 1e140 by step 450); CFD1 harness knobs (stall, step cap, trace)

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-cfd/tests/overset_cfd1.rs

@@ -123,7 +123,10 @@ async fn run_cfd1(ny: usize) -> CfdResult<Cfd1> {
     // progress (the noise floor); measured at ny = 41 without it: 5.0 rounds
     // per corrector on average (the second corrector 8 every step), 2099 s.
     let params = OversetParameters {
-        stall_rounds: 2,
+        stall_rounds: std::env::var("RTX_OVERSET_STALL")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(2),
         ..OversetParameters::default()
     };
     let mut solver = OversetPisoSolver::new(background, patch, (nx, ny, h, h), params)?;
@@ -165,9 +168,42 @@ async fn run_cfd1(ny: usize) -> CfdResult<Cfd1> {
     let mut steps = 0;
     let mut rounds_total = 0usize;
     let mut correctors_total = 0usize;
+    let max_steps: usize = std::env::var("RTX_OVERSET_CFD1_MAX_STEPS")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(2_000_000);
+    let trace_first = std::env::var("RTX_OVERSET_CFD1_TRACE").is_ok();
     loop {
         let r = solver.advance(&mut field, dt).await?;
         steps += 1;
+        let every: usize = std::env::var("RTX_OVERSET_CFD1_TRACE_EVERY")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(0);
+        if (trace_first && steps <= 6) || (every > 0 && steps % every == 0) {
+            let pmax = field
+                .background
+                .p
+                .iter()
+                .fold(0.0_f64, |m, v| m.max(v.abs()));
+            let ppmax = field.patch.p.iter().fold(0.0_f64, |m, v| m.max(v.abs()));
+            let upmax = field.patch.u.iter().fold(0.0_f64, |m, v| m.max(v.abs()));
+            println!(
+                "      step {steps}: rounds {:?} converged {} stalled {} bg res {:.2e} patch div {:.2e} patch iters {} conv {} | max|p| bg {pmax:.3e} patch {ppmax:.3e} max|u| patch {upmax:.3e} | defect bg {:.2e} patch {:.2e}",
+                r.rounds,
+                r.schwarz_converged,
+                r.schwarz_stalled,
+                r.background_residual,
+                r.patch_max_divergence,
+                r.patch_poisson_iterations,
+                r.patch_converged,
+                r.background_mass_defect,
+                r.patch_mass_defect
+            );
+        }
+        if steps >= max_steps {
+            break;
+        }
         rounds_total += r.rounds.iter().sum::<usize>();
         correctors_total += r.rounds.len();
         if steps % 50 == 0 {