rtx-fsi: spike-clamp the force-measurement probe

measure_force integrated raw traction samples while the coupling loads
carried the 20x-median clamp — so the s = 1 benchmark run's REPORTED
drag/lift were +-4,000-scale garbage against a +-78 reference while its
displacements matched the benchmark to 0.1%. Collect, clamp, then
integrate, both probes. Reporting only: the committed FSI2 default
reproduces its trajectory to every printed digit (uy 3.7732 / 3.7920
mm) and its rigid-phase drag (121.4) with the clamp in.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq

crates/specialized/rtx-fsi/tests/fsi2_harness/mod.rs

@@ -449,8 +449,13 @@ impl Fsi2Harness {
         let mask = solver.mask().unwrap();
         let body = solver.body().unwrap();
         let vertices = self.shared.read().unwrap().0.clone();
-        let mut drag = 0.0;
-        let mut lift = 0.0;
+        // Collect, then clamp, then integrate: the same 20x-median spike
+        // clamp the coupling loads carry. Without it the REPORTED
+        // drag/lift at large deformation are dominated by the rare wild
+        // reconstructions (the s = 1 benchmark run printed +-4,000-scale
+        // load swings against a +-78 reference while its displacements
+        // matched the benchmark to 0.1%).
+        let mut samples: Vec<(f64, f64, f64)> = Vec::new();
         let poly_probe = EmbeddedBody::polygon(vertices.clone());
         for s in poly_probe.surface_samples(0.5 * self.h) {
             if circle_sdf(s.x, s.y) < 1e-9 {
@@ -459,8 +464,7 @@ impl Fsi2Harness {
             if let Some((tx, ty)) = mask.traction_at(
                 body, &field.u, &field.v, &field.p, self.mu, 0.0, s.x, s.y, s.nx, s.ny,
             ) {
-                drag += tx * s.ds;
-                lift += ty * s.ds;
+                samples.push((tx, ty, s.ds));
             }
         }
         let circle_probe = EmbeddedBody::circle(0.2, 0.2, 0.05);
@@ -471,10 +475,28 @@ impl Fsi2Harness {
             if let Some((tx, ty)) = mask.traction_at(
                 body, &field.u, &field.v, &field.p, self.mu, 0.0, s.x, s.y, s.nx, s.ny,
             ) {
-                drag += tx * s.ds;
-                lift += ty * s.ds;
+                samples.push((tx, ty, s.ds));
             }
         }
+        let mut magnitudes: Vec<f64> = samples
+            .iter()
+            .map(|(tx, ty, _)| (tx * tx + ty * ty).sqrt())
+            .collect();
+        magnitudes.sort_by(|a, b| a.partial_cmp(b).unwrap());
+        let median = magnitudes.get(magnitudes.len() / 2).copied().unwrap_or(0.0);
+        let cap = 20.0 * median;
+        let mut drag = 0.0;
+        let mut lift = 0.0;
+        for (tx, ty, ds) in samples {
+            let norm = (tx * tx + ty * ty).sqrt();
+            let scale = if median > 0.0 && norm > cap {
+                cap / norm
+            } else {
+                1.0
+            };
+            drag += tx * scale * ds;
+            lift += ty * scale * ds;
+        }
         (drag, lift)
     }