R8-b: pin the free-lateral-face path (CSM1 strip, span 0.02)

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

crates/specialized/rtx-fea/tests/flag3d_structure.rs

@@ -355,6 +355,34 @@ fn plane_strain_3d_reproduces_the_2d_csm1() {
     assert!(rel(uy2, -66.1029e-3) < 0.02 && rel(ux2, -7.18777e-3) < 0.04);
 }
 
+/// The free-lateral-face path, pinned: a narrow strip (span 0.02 = the
+/// thickness) under CSM1 gravity. Measured with `flag3d_csm1_table`
+/// (R8-b, 2026-09-25): u_y(A) = −76.340e-3 — softer than plane strain
+/// (−65.141e-3) because the free faces relax the spanwise stress.
+#[test]
+fn free_lateral_faces_csm1_strip_pin() {
+    let tip = static_3d_csm1(
+        Flag3dSpec::turek_hron(0.02, -0.01, 35, 2, 1),
+        LateralFaces::Free,
+        5,
+    );
+    println!(
+        "  CSM1 35x2x1 span 0.02 free faces: u(A) = ({:.6e}, {:.6e}, {:.2e})",
+        tip.a.x, tip.a.y, tip.a.z
+    );
+    assert!(rel(tip.a.y, -76.340_06e-3) < 1e-5, "uy {}", tip.a.y);
+    assert!(rel(tip.a.x, -9.680_464e-3) < 1e-5, "ux {}", tip.a.x);
+    assert!(
+        tip.a.z.abs() < 1e-12,
+        "mid-span must not move in z: {}",
+        tip.a.z
+    );
+    assert!(
+        (tip.side_low.y - tip.side_high.y).abs() < 1e-12,
+        "span symmetry"
+    );
+}
+
 fn csm3_2d(dt: f64) -> (NonlinearDynamicAnalysis, NodeId) {
     let mesh = quad8_flag(35, 2);
     let a = point_2d(&mesh, 0.6, 0.2);