embedded3 S2-2b (host lever): narrow-band φ re-evaluation for moving bodies (max_surface_speed) — bit-identical, the flag body's ny 62 rebuild 3.54 → 0.30 s; the flag driver uses it

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

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

@@ -108,7 +108,20 @@ pub(super) struct CvGeometry {
 impl Mask {
     /// Classify the grid against `body` at `t` by its cut geometry.
     pub fn build_cut(body: &Body, g: Grid, t: f64, b: Boundaries) -> Result<Self, String> {
-        let cut = CutGeometry::build(body, g, t);
+        Self::build_cut_from(body, g, t, b, None)
+    }
+
+    /// As [`Self::build_cut`], re-evaluating φ only within `band` of the
+    /// previous geometry moved by at most `motion` (see
+    /// [`CutGeometry::build_from`]).
+    pub fn build_cut_from(
+        body: &Body,
+        g: Grid,
+        t: f64,
+        b: Boundaries,
+        prev: Option<(&CutGeometry, f64, f64)>,
+    ) -> Result<Self, String> {
+        let cut = CutGeometry::build_from(body, g, t, prev);
         let (nx, ny, nz) = (g.nx, g.ny, g.nz);
         let periodic = b.z0 == Side::Periodic;
         let allowed = |side: Side| matches!(side, Side::Velocity | Side::Periodic | Side::SlipWall);