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/src/solvers/incompressible/embedded3/loads.rs

@@ -83,7 +83,10 @@ impl Mask {
                 }
             }
         }
-        if nodes.iter().all(|&(idx, _)| self.is_fluid_cell(idx)) {
+        // A virtually merged small cell carries its master's pressure
+        // unknown, not its own: it is dropped from the fit.
+        let usable = |idx: usize| self.is_fluid_cell(idx) && self.master(idx).is_none();
+        if nodes.iter().all(|&(idx, _)| usable(idx)) {
             return Some(nodes.iter().map(|&(idx, w)| w * p[idx]).sum());
         }
         // Weighted by the z-direction weight (a z-invariant field then fits
@@ -96,7 +99,7 @@ impl Mask {
             for _ in 0..dirs(ny, fy).len() * dirs(nx, fx).len() {
                 let &(idx, _) = it.next().expect("node");
                 let _ = dk;
-                if self.is_fluid_cell(idx) {
+                if usable(idx) {
                     let (k, j, i) = g.kji(idx);
                     pts.push((
                         (i as f64 + 0.5) * dx,