Fix edge_edge_mollifier_gradient_wrt_x to use ∇_rest ε_x

[Huangzizhou]

Summary

edge_edge_mollifier_gradient_wrt_x returns the gradient of the EE parallel mollifier with respect to rest positions, computed as ∇_rest m = ∂m/∂ε · ∇_rest ε (since m depends on rest only through eps_x). The second factor was using edge_edge_mollifier_gradient(positions, eps_x) (returns ∇_position m) instead of edge_edge_mollifier_threshold_gradient(rest) (returns ∇_rest ε).

Sister function edge_edge_mollifier_gradient_jacobian_wrt_x already used the correct factor; this restores consistency between the two.

 if (ee_cross_norm_sqr < eps_x) {
     return edge_edge_mollifier_derivative_wrt_eps_x(
                ee_cross_norm_sqr, eps_x)
-        * edge_edge_mollifier_gradient(ea0, ea1, eb0, eb1, eps_x);
+        * edge_edge_mollifier_threshold_gradient(
+              ea0_rest, ea1_rest, eb0_rest, eb1_rest);
 }

Why it matters

The existing IPC shape_derivative test (test_barrier_potential.cpp:248) uses a stacked-cube scene that doesn't trigger the mollifier branch, so the bug was invisible to existing coverage. It surfaces on near-parallel EE pairs near dhat — in a polyfem shape-optimization transient adjoint, this produced a ~400× per-pair inflation and an ~10⁸× sign-flipped end-to-end gradient.

Test plan

New regression test tests/src/tests/potential/ipc_test_ee_near_parallel_shape_derivative.cpp constructs a 4-vertex / 2-edge scene with a single mollified EE pair (y_sep = 9.99e-4, z_twist ∈ {5e-3, 1e-2, 2e-2}) and FD-checks the full 12×12 analytic shape_derivative against centered FD of gradient at fixed collision set (same convention as test_barrier_potential.cpp:377-391).

• Without the fix: rel ≈ 1.4–2.6×10⁻² (test FAILS).
• With the fix: rel ≈ 3–5×10⁻⁴ (test PASSES). Threshold is 5×10⁻³.

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