embedded3 S2-5: centroid diffusion default ON (RTX_E3_DIFFUSION_CENTROID=0 restores the records); flat_wall_position_is_second_order gate; closure.rs split (cutwall.rs under 700)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/tests/embedded3_wall_position.rs

@@ -53,10 +53,8 @@ fn offset(ny: usize, theta: f64) -> (f64, Vec<(f64, f64, f64)>) {
     for k in 0..=nz {
         for j in 0..ny {
             for i in 0..nx {
-                if k < nz || true {
-                    let idx = g.wface(k.min(nz), j, i);
-                    field.w[idx] = exact((j as f64 + 0.5) * h);
-                }
+                let idx = g.wface(k, j, i);
+                field.w[idx] = exact((j as f64 + 0.5) * h);
             }
         }
     }
@@ -100,3 +98,24 @@ fn flat_wall_effective_position() {
         }
     }
 }
+
+/// The gate: with the centroid diffusion (the default) the effective wall
+/// position is second order — within 0.08 h at ny 16, halving in units of
+/// h at ny 32, the same at every cut fraction (before S2-5: −½θ h at
+/// every resolution).
+#[test]
+fn flat_wall_position_is_second_order() {
+    if std::env::var("RTX_E3_DIFFUSION_CENTROID").is_ok_and(|v| v == "0") {
+        return;
+    }
+    for theta in [0.25, 0.75] {
+        let (coarse, _) = offset(16, theta);
+        let (fine, _) = offset(32, theta);
+        println!("  θ {theta}: offset {coarse:+.4} h at ny 16, {fine:+.4} h at ny 32");
+        assert!(coarse.abs() < 0.08, "ny 16 offset {coarse}");
+        assert!(
+            fine.abs() < 0.6 * coarse.abs(),
+            "the offset does not halve: {coarse} → {fine}"
+        );
+    }
+}