embedded3 oblique instrument: RTX_E3_OBLIQUE_DRIVE=pressure (no body force; the sides pin the flow rate)

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

crates/specialized/rtx-cfd/tests/embedded3_wall_position_oblique.rs

@@ -113,8 +113,13 @@ fn reading(n: usize, slope: f64, c0: f64, along_z: bool, s26: Option<bool>) -> R
             (s * tx, s * ty, 0.0)
         }
     });
+    // `RTX_E3_OBLIQUE_DRIVE=pressure`: no body force — the sides' exact
+    // profile pins the flow rate and the pressure slope alone drives it.
+    let by_pressure = std::env::var("RTX_E3_OBLIQUE_DRIVE").is_ok_and(|v| v == "pressure");
     solver.set_momentum_source(move |_, _, _, _| {
-        if along_z {
+        if by_pressure {
+            (0.0, 0.0, 0.0)
+        } else if along_z {
             (0.0, 0.0, F)
         } else {
             (F * tx, F * ty, 0.0)