feat(num): N3 selectable scale-floor clamp mode (HardJava | SmoothBarrier)

The hyper-ideal vertex scale b is floored to keep the geometry valid.  The
original clamp `b<0 → 0.01` mirrors the Java oracle but is only C⁰ (in fact
value-discontinuous at b=0): a Newton step crossing the feasibility boundary
hits a kink that can stall convergence (numerical-stability audit N3).

Rather than replace the Java-faithful behaviour (which would break the golden
parity tests), make the floor a selectable mode so BOTH the Java standpoint
and the clean mathematics are available:

- HyperIdealScaleClamp::HardJava (DEFAULT) — the original snap, bit-for-bit
  faithful to HyperIdealFunctional.java → all parity tests unchanged.
- HyperIdealScaleClamp::SmoothBarrier — C¹ softplus floor
  b ↦ floor + softplus_β(b−floor), β = HYPER_IDEAL_SCALE_SHARPNESS (=100);
  ≈ identity away from the floor, smooth across b=0.  Opt-in.

clamp_hyper_ideal_scale centralises the logic (also folds in the N4 nachzügler:
compute_face_angles used a bare 0.01).  The mode threads with a defaulted
trailing parameter through compute_face_angles, face_angles_from_local_dofs,
evaluate_hyper_ideal, the four hyper_ideal_hessian* variants and
newton_hyper_ideal — so every existing call site keeps HardJava behaviour.

Tests (+4): clamp-function C¹/floor/identity contract, mode-equivalence away
from the boundary, and end-to-end SmoothBarrier convergence to the same Java
golden vector (LawsonHyperIdeal).  296/296 CGAL tests pass.
Documented in doc/math/geometry-modes.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

code/tests/cgal/test_hyper_ideal_hessian.cpp

@@ -338,3 +338,81 @@ TEST(HyperIdealHessian, BlockFD_FasterThanFullFD)
     EXPECT_GE(ms_full, 3 * ms_block)
         << "Block-FD should be at least 3× faster than full-FD on this mesh";
 }
+
+// ════════════════════════════════════════════════════════════════════════════
+// N3 — Scale-floor clamp modes (HardJava vs SmoothBarrier)
+//
+// The vertex-scale floor can be applied two ways (HyperIdealScaleClamp):
+//   • HardJava (default): b<0 → floor.  Faithful to Java, but only C⁰ — and
+//     in fact value-discontinuous at b=0 (jumps from `floor` to 0).
+//   • SmoothBarrier: floor + softplus_β(b−floor).  C¹ everywhere, ≈ b away
+//     from the floor, and keeps b ≥ floor > 0 smoothly.
+// These tests pin the contract of both modes and the default-preservation.
+// ════════════════════════════════════════════════════════════════════════════
+
+TEST(HyperIdealClamp, SmoothBarrierIsContinuousAndC1AcrossZero)
+{
+    const auto Hard   = HyperIdealScaleClamp::HardJava;
+    const auto Smooth = HyperIdealScaleClamp::SmoothBarrier;
+    auto f = [](double b, HyperIdealScaleClamp mode) {
+        return clamp_hyper_ideal_scale(b, /*variable=*/true, mode);
+    };
+
+    const double eps = 1e-6;
+
+    // HardJava jumps in VALUE at b=0 (floor on the left, 0 on the right).
+    EXPECT_GT(std::abs(f(-eps, Hard) - f(+eps, Hard)), 0.5 * HYPER_IDEAL_SCALE_FLOOR);
+
+    // SmoothBarrier is continuous in value across b=0 …
+    EXPECT_NEAR(f(-eps, Smooth), f(+eps, Smooth), 1e-4);
+
+    // … and C¹: the one-sided slopes across b=0 agree (the HardJava slopes,
+    // 0 on the left and 1 on the right, would not).
+    auto slope = [&](double b0, HyperIdealScaleClamp mode) {
+        return (f(b0 + eps, mode) - f(b0 - eps, mode)) / (2.0 * eps);
+    };
+    const double sL = slope(-1e-3, Smooth);
+    const double sR = slope(+1e-3, Smooth);
+    EXPECT_NEAR(sL, sR, 5e-2) << "SmoothBarrier derivative should be continuous";
+}
+
+TEST(HyperIdealClamp, SmoothBarrierStaysAboveFloorAndApproachesIdentity)
+{
+    const auto Smooth = HyperIdealScaleClamp::SmoothBarrier;
+    auto f = [&](double b) { return clamp_hyper_ideal_scale(b, true, Smooth); };
+
+    // At or above the floor for any input (mathematically > floor; for very
+    // negative b the softplus underflows to 0 in double, giving exactly floor).
+    EXPECT_GE(f(-1e6), HYPER_IDEAL_SCALE_FLOOR);
+    EXPECT_GE(f(-1.0), HYPER_IDEAL_SCALE_FLOOR);
+    EXPECT_GT(f(0.0),  0.0);
+    EXPECT_LT(f(-1e6) - HYPER_IDEAL_SCALE_FLOOR, 1e-9);   // converges down to floor
+
+    // ≈ identity well above the floor (the normal operating regime).
+    EXPECT_NEAR(f(1.0),  1.0,  1e-9);
+    EXPECT_NEAR(f(5.0),  5.0,  1e-12);
+
+    // Pinned DOFs are never clamped, regardless of mode.
+    EXPECT_EQ(clamp_hyper_ideal_scale(-3.0, /*variable=*/false, Smooth), -3.0);
+}
+
+// Away from the feasibility boundary (all b = 1 ≫ floor), the two modes must
+// produce the same Hessian — SmoothBarrier only differs near b ≈ floor, so the
+// default-mode parity results are not perturbed in the normal regime.
+TEST(HyperIdealClamp, ModesAgreeAwayFromBoundary)
+{
+    auto mesh = make_tetrahedron();
+    auto m    = setup_hyper_ideal_maps(mesh);
+    const int n = assign_all_dof_indices(mesh, m);
+    auto x = natural_x(mesh, m);   // b = 1, a = 0.5 — all well above the floor
+
+    auto H_hard = hyper_ideal_hessian_block_fd_sym(
+        mesh, x, m, 1e-5, HyperIdealScaleClamp::HardJava);
+    auto H_soft = hyper_ideal_hessian_block_fd_sym(
+        mesh, x, m, 1e-5, HyperIdealScaleClamp::SmoothBarrier);
+
+    Eigen::MatrixXd Dh(H_hard), Ds(H_soft);
+    EXPECT_LT((Dh - Ds).cwiseAbs().maxCoeff(), 1e-9)
+        << "clamp modes must agree when every scale is well above the floor";
+    (void)n;
+}

code/tests/cgal/test_lawson_hyperideal.cpp

@@ -194,6 +194,54 @@ TEST(LawsonHyperIdeal, ConvergenceGoldenVector_JavaXVal)
     }
 }
 
+// ════════════════════════════════════════════════════════════════════════════
+// N3 — SmoothBarrier clamp mode converges to the same golden solution
+//
+// The C¹ smooth-barrier scale floor is an opt-in alternative to the Java hard
+// clamp.  On this well-posed problem the scales stay well above the floor
+// throughout the solve, so the barrier is ≈ identity and the converged vector
+// must match the same golden classes as the HardJava run above — demonstrating
+// the smooth mode is a drop-in that does not move the solution when the clamp
+// region is never entered.
+// ════════════════════════════════════════════════════════════════════════════
+TEST(LawsonHyperIdeal, SmoothBarrierConvergesToGoldenVector)
+{
+    std::set<Edge_index> original;
+    ConformalMesh m = make_lawson_square_tiled(&original);
+
+    HyperIdealMaps maps = setup_hyper_ideal_maps(m);
+    const int n = assign_all_dof_indices(m, maps);
+    ASSERT_EQ(n, 4 + 18);
+
+    for (auto e : m.edges())
+        if (original.count(e)) maps.theta_e[e] = PI / 2.0;
+
+    std::vector<double> x0(static_cast<std::size_t>(n), 1.0);
+    auto res = newton_hyper_ideal(m, x0, maps, /*tol=*/1e-10, /*max_iter=*/200,
+                                  /*hess_eps=*/1e-5,
+                                  HyperIdealScaleClamp::SmoothBarrier);
+    ASSERT_TRUE(res.converged)
+        << "SmoothBarrier HyperIdeal Newton did not converge; ||G||="
+        << res.grad_inf_norm;
+
+    constexpr double b_gold      = 1.1462158341786262;
+    constexpr double a_orig_gold = 1.7627471737467797;
+    constexpr double a_aux_gold  = 2.633915794495759;
+    const double tol = 1e-5;
+
+    for (auto v : m.vertices()) {
+        const int iv = maps.v_idx[v];
+        ASSERT_GE(iv, 0);
+        EXPECT_NEAR(res.x[static_cast<std::size_t>(iv)], b_gold, tol);
+    }
+    for (auto e : m.edges()) {
+        const int ie = maps.e_idx[e];
+        ASSERT_GE(ie, 0);
+        const double gold = original.count(e) ? a_orig_gold : a_aux_gold;
+        EXPECT_NEAR(res.x[static_cast<std::size_t>(ie)], gold, tol);
+    }
+}
+
 // ════════════════════════════════════════════════════════════════════════════
 // Branch-points variant — Java HyperIdealConvergenceTest...WithBranchPoints
 //