Sobol sensitivity analysis in screen()

[jc-macdonald]

Problem

screen() only supports Morris screening. Sobol variance-based sensitivity indices are the standard next step for quantitative factor importance — SALib already supports them.

Proposed

• Add method="sobol" to screen()
• Use SALib.sample.sobol + SALib.analyze.sobol (already a dependency)
• Return S1 (first-order) indices in the same format as Morris mu_star
• Optionally return ST (total-order) indices

Notes

SALib is already a dependency. The implementation pattern mirrors the existing Morris path — mostly plumbing.