PERF-2: red-black symmetric Gauss–Seidel smoother behind a knob (MgSmoother::RedBlack; default lexicographic = the recorded regime, bit-identical) — red/black cell lists per level, the symmetric pair red,black,black,red, the operator cache keyed on the smoother; EmbeddedParameters::poisson_smoother, harness knobs RTX_FSI2O_MG_RB (overset) and RTX_FSI2_MG_RB (embedded, the noise probe); pin: solves the masked problem to the same stop, agrees with lexicographic to 2e-12, cached = uncached bit for bit

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

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

@@ -71,6 +71,9 @@ pub struct EmbeddedParameters {
     /// and the stop stay f64; only the preconditioner runs in single
     /// precision. No effect with [`PoissonSolverKind::Sor`].
     pub poisson_precision: MgPrecision,
+    /// The multigrid smoother ordering (PERF-2; default lexicographic, the
+    /// recorded regime).
+    pub poisson_smoother: super::poisson::MgSmoother,
     /// Convective face values in the explicit predictor (default
     /// [`ConvectionScheme::Upwind`], which is bit-identical to the fixed-grid
     /// PISO). The TVD schemes add SIMPLE's limited correction to each
@@ -93,6 +96,7 @@ impl Default for EmbeddedParameters {
             boundaries: AleBoundaries::default(),
             poisson_solver: PoissonSolverKind::Sor,
             poisson_precision: MgPrecision::F64,
+            poisson_smoother: super::poisson::MgSmoother::Lexicographic,
             convection_scheme: ConvectionScheme::Upwind,
         }
     }
@@ -215,6 +219,12 @@ impl EmbeddedPisoSolver {
         self.parameters.poisson_precision = precision;
     }
 
+    /// The multigrid smoother ordering (PERF-2's red-black knob); the
+    /// cached operator is rebuilt on the next solve.
+    pub fn set_poisson_smoother(&mut self, smoother: super::poisson::MgSmoother) {
+        self.parameters.poisson_smoother = smoother;
+    }
+
     /// Mask hysteresis for the moving-body rebuild, as a fraction of the
     /// min cell size (default 0, exactly the plain rebuild). With a band,
     /// a cell within `band * h_min` of the surface keeps the