Classic RK4 via the shared kernel. Whole-array flat update (halos included) — legal under the halo-zero resid invariant; halos are refreshed inside compute_resid_2d before each use. New in R2; no bit-history to preserve.
| Type | Intent | Optional | Attributes | Name | ||
|---|---|---|---|---|---|---|
| type(solver_state_2d_t), | intent(inout), | target | :: | state |