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/src/solvers/incompressible/embedded3/step/mod.rs

@@ -121,7 +121,7 @@ pub struct Parameters {
     /// wall's in-plane normal) instead of `h`. With `h` the coupling of a
     /// cut face to its neighbour is weak by `(1 + α)/2` and the no-slip
     /// surface sits `½(1 − α) h` inside the body (the flat-wall instrument).
-    /// `RTX_E3_DIFFUSION_CENTROID=1`.
+    /// Default ON; `RTX_E3_DIFFUSION_CENTROID=0` restores the old spacing.
     pub diffusion_centroid: bool,
 }
 
@@ -147,7 +147,9 @@ impl Default for Parameters {
                 .is_ok_and(|v| v == "oblique"),
             wall_exchange_axis: std::env::var("RTX_E3_WALL_EXCHANGE").is_ok_and(|v| v == "axis"),
             pressure_centroid: std::env::var("RTX_E3_PRESSURE_CENTROID").is_ok_and(|v| v == "1"),
-            diffusion_centroid: std::env::var("RTX_E3_DIFFUSION_CENTROID").is_ok_and(|v| v == "1"),
+            // ON by default since S2-5 (`=0` reproduces the records before it).
+            diffusion_centroid: std::env::var("RTX_E3_DIFFUSION_CENTROID")
+                .map_or(true, |v| v != "0"),
         }
     }
 }