Symmetrize Hessian in-place during setup (0.5*(H+H'))

[Copilot]

• Understand current code in daqp_update_Rinv
• Move diagonal check before the packing step; for !is_factored scan upper-triangle of raw H directly (O(n²/2) reads, no writes)
• In diagonal fast path read H[i*n+i] directly (no Rinv buffer needed)
• Move packing step to after diagonal check, only reached for the dense Cholesky path
• Move RinvD→Rinv restore guard to the dense path (not at function entry), matching v0.8.2's cleaner pointer management
• Replace 1e-12 magic literals with zero_tol for consistency
• Build and run all 21 Python tests (pass)
• Benchmark diagonal H (NPROB=100, NREP=20): no regression vs v0.8.2 — current is ≤1% different at small n and −6 to −12% faster at n=50..200
• Code review addressed (pointer swap, magic numbers)
• CodeQL scan: 0 alerts

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.