M4 — Symmetric indefinite LDLᵀ/LDLᴴ (MadNLP-ready)

Definition of done (PLAN.md §5): In-front Bunch–Kaufman, pivot_threshold, pivot_epsilon(_alg), pivot_sign, inertia, pivot_stats, npivots, diag, solve_diag, ldlt/ldlt!; vendor sytrf with post-check on root fronts. MadNLPGPU gets a SparseDirectSolver option; validated on OPF/ExaModels KKTs (MadNLP K2/K2r, MadIPM, MadNCL settings) against the cuDSS path, including refinement counts against MA27/MA57.

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