Fix Eigen interface: general Hessian workspace allocation and add time_limit setting

[Copilot]

The DAQP workspace class was broken for non-identity Hessians because allocate_daqp_ldp was called with alloc_R=0, alloc_v=0, leaving work_.Rinv = nullptr. Any call to update(H, ...) with a non-identity H would crash inside daqp_update_Rinv writing to a null pointer. Additionally, the default update mask always included DAQP_UPDATE_Rinv even when H was null, causing a null dereference in the other direction.

Changes

• Pre-allocate Rinv/v buffers — allocate_daqp_ldp now called with alloc_R=1, alloc_v=1. Two new private members rinv_buf_ / v_buf_ hold the raw pointers.

• Buffer visibility management in update() — daqp.c uses Rinv != nullptr to detect non-identity Hessian. The buffers are exposed (work_.Rinv = rinv_buf_) only when H is provided, and hidden (nullptr) otherwise to preserve correct LDP/identity-Hessian behavior:

  DAQP solver(n, m, ns);
  solver.update(H, f, A, bu, bl, sense, bp);  // now works for any H
  solver.solve(A, bu, bl, bp);                 // LDP path still correct

• Fix default update mask — DAQP_UPDATE_Rinv / DAQP_UPDATE_v are only added to the default mask when H / f are actually provided.

• Destructor cleanup — restores buffer pointers into work_.Rinv before free_daqp_ldp to avoid leaking the pre-allocated memory when in LDP mode (where both Rinv and RinvD are null).

• Add set_time_limit(double val) — mirrors all other settings setters; was the only DAQPSettings field without a corresponding setter in the Eigen interface.

• New test 06_general_hessian — validates the workspace class with a non-identity H, comparing against the stateless daqp_solve() reference.

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