rtx-cfd: overset Schwarz stall detection is opt-in (stall_rounds, default 0): right for a steady march (n=64: 2.26 rounds mean, no cap hits, L2 identical), wrong for a transient (falsifier max spike 594 → 4044 N/m when on); the MMS harness sets 2

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

crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs

@@ -52,6 +52,14 @@ pub struct OversetParameters {
     /// is pinned only through the fringe and decays weakly across the
     /// overlap (measured: 4.5e-3 relative after 20 rounds).
     pub anderson_depth: usize,
+    /// Stall detection: stop a corrector's rounds when the exchanged change
+    /// has not fallen by 30% over this many rounds (0 = off, the default).
+    /// Right for a steady march, where the step's `p'` sits at the inner
+    /// solvers' noise floor and rounds cannot help (at n = 64/128 the cap
+    /// was burned on 4% of the steps; a 7.5 h n = 128 march); WRONG for a
+    /// transient — on the falsifier plate it stopped rounds that were still
+    /// converging and raised the max force spike from 594 to 4044 N/m.
+    pub stall_rounds: usize,
     /// Patch rows below the acceptor row kept non-hole
     /// (`overlap::DEFAULT_OVERLAP_ROWS`).
     pub overlap_rows: usize,
@@ -64,6 +72,7 @@ impl Default for OversetParameters {
             schwarz_tolerance: 1e-3,
             max_rounds: 20,
             anderson_depth: 3,
+            stall_rounds: 0,
             overlap_rows: overlap::DEFAULT_OVERLAP_ROWS,
         }
     }
@@ -509,13 +518,14 @@ impl OversetPisoSolver {
                     done = true;
                     break;
                 }
-                // Stall: no progress over three rounds means the exchanged
+                // Stall: no progress over two rounds means the exchanged
                 // values sit at the inner solvers' noise floor (the step's
                 // own p' is that small near a steady state); more rounds
                 // cannot help — measured as 6560 of 150k steps burning the
                 // 20-round cap at n = 64, and a 7.5 h n = 128 march.
                 history.push(change);
-                if history.len() >= 4 && change > 0.7 * history[history.len() - 4] {
+                let k = self.params.stall_rounds;
+                if k > 0 && history.len() > k && change > 0.7 * history[history.len() - 1 - k] {
                     schwarz_stalled += 1;
                     done = true;
                     break;