ENH add gradient Linfty norm to objective

It's a standard convergence criterion for smooth optimization

The issue identifies the benchmark objective as the entry point but names no file or test. Inspect the repository's objective implementation and existing convergence criteria first, then determine where the gradient Linfty norm belongs. Done means the objective evaluates this standard smooth-optimization criterion, with the behavior covered by the repository's existing validation.

Written by the indexing model from the issue text.