Lowstorage 3s

[singhharsh1708]

Summary

Converts the 3S family (ParsaniKetchesonDeconinck3S32/82/53/173/94/184/105/205) to use plain-dispatch _perform_step_oop! / _perform_step_iip!(integrator, cache, tab::LowStorageRK3SConstantCache) in generic_low_storage_perform_step.jl. The LowStorageRK3SConstantCache struct is unchanged; only the step bodies move to the generic file.

Thin wrappers in low_storage_rk_perform_step.jl delegate to the generic implementations.

Part of the stacked series: #3683 (2N) → #3684 (2C) → #3685 (3S) → #3686 → #3687 → #3688.

Test plan

• OOP and IIP paths validated against master — step counts and solution values identical.
• CI passes.

[singhharsh1708]

Closing in favour of the consolidated approach in #3683 — the substantive change (consolidating RK46NL into LowStorageRKTableau{TwoN}) will be done there instead.