Lowstorage 2c

[singhharsh1708]

Summary

Converts the 2C family (CFRLDDRK64, TSLDDRK74) to use LowStorageRKTableau{:two_c} in place of the deleted LowStorageRK2CConstantCache.

Adds _perform_step_oop! / _perform_step_iip! dispatch for :two_c in generic_low_storage_perform_step.jl. Thin wrappers in low_storage_rk_perform_step.jl delegate to the generic implementations.

Part of the stacked series: #3683 (2N) → #3684 (2C) → #3685 → #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.