Clippy Lints: clippy::indexing_slicing

[panayang]

We all know that in many numerical computing cases, performance
and security must be balanced. Direct indexing is structurally safe
in our validated internal loops, and using it allows the compiler to
perform Bounds Check Elision (BCE), which is critical for performance
in hot loops. The long-term plan is to refactor these loops to use
Ghost Cells or clear range iteration to make the safety provable to Clippy
without suppression. And we are here to gather your ideas on this issue.