fix(scf): FullPivLU has no info() — use isInvertible() for the DIIS guard

The DIIS coefficient solve in solve_scf checked `lu.info() == Eigen::Success`
on an Eigen::FullPivLU, which has no info() member.  It slipped through local
builds (newer system Eigen found via find_package) but breaks the CI wheel
build against the pinned Eigen 3.4.0 (the FetchContent fallback):

  solver_scf.hpp:299: 'Eigen::FullPivLU<...>' has no member named 'info'

Introduced by ddee430 (experimental SCF solvers), after v1.0.3 — so CI never
exercised it until the pipeline was re-armed.  isInvertible() is the correct,
portable check: reject the DIIS extrapolation when the residual overlap matrix
B is singular and fall back to the plain step.  SCF regression tests green.

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

Author: skilledwolf

cpp/include/cpp_hf/solver_scf.hpp

@@ -296,7 +296,9 @@ inline SCFResult solve_scf(const HFKernel& K, const c64* P0, f32 n_e,
                 {
                     Eigen::FullPivLU<decltype(B)> lu(B);
                     sol = lu.solve(rhs);
-                    diis_ok = lu.info() == Eigen::Success;
+                    // FullPivLU has no info(); isInvertible() is the right check
+                    // (reject DIIS when the residual matrix B is singular).
+                    diis_ok = lu.isInvertible();
                     for (std::size_t i = 0; i < m; ++i)
                         diis_ok = diis_ok && std::isfinite(static_cast<double>(sol[i]));
                 }