Determine the optimal treatment of generalized step indices in Iris-Lean

Determine how Iris-Lean should support generalized step indices while best balancing concision, performance, and robustness.

Background

Iris-Lean provides a complete port of the core Iris library relative to the referenced Iris-Rocq revision, but generalized step indices are explicitly excluded. The unresolved issue concerns selecting an implementation that offers an appropriate balance among concise definitions and proofs, efficient execution, and robust behavior.