embedded3 S2-1: the traction sampler on the cut field (pressure fit excludes merged cells; sampler route in the sphere MMS and DFG drivers)

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

crates/specialized/rtx-cfd/tests/embedded3_embedded_mms.rs

@@ -54,8 +54,13 @@ fn ladder(resolutions: &[usize], scheme: WallScheme) -> Ladder {
             mm.force_cv[1] - fcv[1],
             mm.force_cv[2] - fcv[2],
         ]) / f_scale;
+        let a = norm([
+            mm.force_sampler[0] - fe[0],
+            mm.force_sampler[1] - fe[1],
+            mm.force_sampler[2] - fe[2],
+        ]) / f_scale;
         println!(
-            "  n = {n:3}  L2 u {:.4e} (order {rate})  max div {:.2e}  ghost corr {:.2e}  F_surface {:.4?} rel {s:.3e} (skipped {})  F_cv {:.4?} rel {c:.3e}",
+            "  n = {n:3}  L2 u {:.4e} (order {rate})  max div {:.2e}  ghost corr {:.2e}  F_surface {:.4?} rel {s:.3e} (skipped {})  F_cv {:.4?} rel {c:.3e}  F_sampler rel {a:.3e}",
             mm.l2_velocity,
             mm.max_div,
             mm.ghost_correction,
@@ -63,6 +68,15 @@ fn ladder(resolutions: &[usize], scheme: WallScheme) -> Ladder {
             mm.skipped,
             mm.force_cv
         );
+        if scheme == WallScheme::CutCell {
+            // S2-1 (reported, not gated): the traction sampler on the cut
+            // field against the box route — 1.15× at n 24 on the first read
+            // (`docs/embedded3_campaign.md`, S2-1).
+            println!(
+                "    sampler / box error ratio at n = {n}: {:.3}",
+                a / c.max(1e-300)
+            );
+        }
         se.push(s);
         ce.push(c);
     }