Lowstorage misc

[singhharsh1708]

Summary

Converts the remaining three families to plain-dispatch step implementations in generic_low_storage_perform_step.jl:

• RK46NL — dispatched on RK46NLConstantCache
• SHLDDRK52 — dispatched on SHLDDRK52ConstantCache
• SHLDDRK_2N — dispatched on SHLDDRK_2NConstantCache (mutable struct with a step::Int field for odd/even stage alternation)

Note: SHLDDRK_2N has a pre-existing difference between IIP and OOP function-evaluation counts (5001 vs 5501) due to its odd/even step alternation design. This is not a regression introduced here — it is identical to master.

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

Final PR in the stacked series: #3683 (2N) → #3684 (2C) → #3685 (3S) → #3686 (3Sp + 3SpFSAL) → #3687 (2RP/3RP/4RP/5RP) → #3688 (RK46NL + SHLDDRK52 + SHLDDRK_2N).

Test plan

• OOP and IIP paths validated against master — step counts and solution values identical (SHLDDRK_2N IIP/OOP nf difference is pre-existing).
• 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.