R8-a harness: save/load of the coupled state (continue on the same grid, or extrude the slab onto the full duct)

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/specialized/rtx-fsi/tests/fsi2_embedded3/fluid.rs

@@ -155,7 +155,19 @@ impl E3Fluid {
     /// thin slab periodic in z (the flag as a 2D problem), 0 the full 0.41 m
     /// duct with slip side walls; the 2D inflow (parabolic in y, Ū 1) in
     /// both. `speed` bounds the flag's surface speed (the narrow band).
-    pub fn build(ny: usize, nz_slab: usize, speed: f64, rest: Line) -> Self {
+    /// `start`: a saved state (its time, its line, its fields — extruded
+    /// onto every plane when the saved nz differs); the rest flow otherwise.
+    pub fn build(
+        ny: usize,
+        nz_slab: usize,
+        speed: f64,
+        rest: Line,
+        start: Option<&super::state::Saved>,
+    ) -> Self {
+        let rest = match start {
+            Some(s) => s.line.clone(),
+            None => rest,
+        };
         let h = H / ny as f64;
         let nx = (L / h).round() as usize;
         let nz = if nz_slab > 0 {
@@ -249,6 +261,10 @@ impl E3Fluid {
                 }
             }
         }
+        if let Some(s) = start {
+            s.fill(&mut field);
+            solver.set_time(s.t);
+        }
         solver.initialize(&mut field);
         let mut device = DeviceStep::new(solver, g);
         device.upload(&field);