embedded3 item 10b: virtual merging of small cells in the projection (fraction < 0.1 → master = largest active face neighbour; off-stencil links on the fine Poisson level; merged rhs, anchor, mass residual and source scale); static sphere rows within 0.02 %, loads 9.4/10.4 %; stadium falsifier spikes 39–54× below the binary wall (circle 6–8×), energy per event 6–9× lower

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

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs

@@ -88,6 +88,10 @@ pub struct Mask {
     /// step (a dying cell empties through the apertures it had); `None` =
     /// the instantaneous kinds.
     pub(super) step_open: Option<(Vec<bool>, Vec<bool>, Vec<bool>, Vec<bool>)>,
+    /// Virtual merging (item 10b): the master cell of every small cell
+    /// (`usize::MAX` = its own row) — a small cell shares its pressure
+    /// unknown with its largest active face neighbour in the projection.
+    pub(super) merge_master: Vec<usize>,
 }
 
 /// The z lattice position of a query: the lower plane index, the upper
@@ -497,9 +501,27 @@ impl Mask {
             cut: None,
             step_apertures: None,
             step_open: None,
+            merge_master: Vec::new(),
         })
     }
 
+    /// The master of a virtually merged small cell.
+    #[inline]
+    #[must_use]
+    pub fn master(&self, idx: usize) -> Option<usize> {
+        match self.merge_master.get(idx) {
+            Some(&m) if m != usize::MAX => Some(m),
+            _ => None,
+        }
+    }
+    #[must_use]
+    pub fn merged_cells(&self) -> usize {
+        self.merge_master
+            .iter()
+            .filter(|&&m| m != usize::MAX)
+            .count()
+    }
+
     // The projection's unknowns (the instantaneous kinds at rest).
     #[inline]
     #[must_use]