embedded3 PERF-3 P1-4 (steps 1–3 + d): the band-persistent operator — CutGeometry marks re-evaluated corners; Mask::changed_cells (touched by either build, dilated by one); Level::patch re-derives the changed rows with the constructor's formulas on the kept level; the fine export patched likewise with the parent map as a parallel per-cell map; components plane by plane with a small union across planes; Problem::link_map (sparse) replaces the per-cell link lists on the per-step path; RTX_E3_BAND_CHECK=1 compares against full rebuilds (passed on the slab) — slab CSV byte-identical at every step, device moving/cg green; ny 124 rebuild block 2,569 → 2,119 ms per step

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

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

@@ -110,6 +110,10 @@ pub struct Mask {
     pub(super) wall_exchange_axis: bool,
     /// S2-7: the momentum control volumes' side apertures from the
     /// interpolant on the sides' own corners (host prototype).
+    /// P1-4: the cells whose operator rows may differ from the previous
+    /// step's (touched by either mask's build, dilated by one), ascending;
+    /// `None` on a wall at rest or before the first step.
+    pub(super) changed_cells: Option<Vec<usize>>,
     pub(super) cv_sides_exact: bool,
     /// S2-7: the quadratic wall gradient's second point at the neighbour's
     /// own centroid distance (host prototype).
@@ -546,6 +550,7 @@ impl Mask {
             wall_advancing: false,
             exchange_convection_off: false,
             wall_exchange_axis: false,
+            changed_cells: None,
             cv_sides_exact: false,
             wall_order2_centroid: false,
             wall_exchange_foot: false,
@@ -573,6 +578,13 @@ impl Mask {
             _ => None,
         }
     }
+    /// The cells whose operator rows may have changed since the previous
+    /// step (P1-4), or `None` when every row must be rebuilt.
+    #[must_use]
+    pub fn changed_cells(&self) -> Option<&[usize]> {
+        self.changed_cells.as_deref()
+    }
+
     #[must_use]
     pub fn merged_cells(&self) -> usize {
         self.merge_master