Add Jacobi and BiCGSTAB OpenMP linear solvers

[shaia]

Completes the OpenMP linear-solver tier (previously only CG, GMRES, and Red-Black SOR), so the parallel projection/pressure paths can select these methods instead of falling back to unavailable.

Both are selectable via poisson_solver_create(method, POISSON_BACKEND_OMP) and follow the existing cg_omp/redblack_omp pattern: per-element updates are identical to the scalar reference, so results agree within reduction rounding (verified by new scalar-vs-OMP consistency tests). BiCGSTAB keeps the scalar solve loop and breakdown checks verbatim, parallelizing only the primitives.

Plain lexicographic SOR stays scalar/SIMD-only: it is inherently sequential, and its parallel form is the existing Red-Black SOR OMP solver.

Also updates the stale BiCGSTAB backend test, which asserted OMP create() returns NULL, to the runtime availability contract.

[Copilot]

Pull request overview

This PR completes the OpenMP (“OMP”) tier for Poisson/linear solvers by adding Jacobi and BiCGSTAB OMP backends, enabling parallel pressure/projection paths to select these methods instead of falling back to unavailable backends.

Changes:

• Add new OpenMP implementations for Jacobi and BiCGSTAB linear solvers and register them in the solver factory/build.
• Add scalar-vs-OMP consistency tests for Jacobi and BiCGSTAB, and update the BiCGSTAB backend-availability test to follow the runtime contract.
• Update documentation (ROADMAP/CHANGELOG) to reflect the completed OMP linear-solver tier and clarify why plain lexicographic SOR remains non-OMP.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/math/test_omp_consistency.c	Adds Jacobi + BiCGSTAB scalar-vs-OMP consistency tests and wires them into the test runner.
tests/math/test_bicgstab.c	Updates backend-availability expectations to match runtime poisson_solver_backend_available() contract.
ROADMAP.md	Marks Jacobi/BiCGSTAB OMP as done and documents why plain SOR remains sequential.
lib/src/solvers/linear/omp/linear_solver_jacobi_omp.c	New Jacobi OpenMP backend (parallel interior sweep, common solve loop).
lib/src/solvers/linear/omp/linear_solver_bicgstab_omp.c	New BiCGSTAB OpenMP backend (OMP-parallel primitives, scalar-identical solve loop).
lib/src/solvers/linear/linear_solver.c	Registers the new OMP solver factories for Jacobi and BiCGSTAB.
lib/src/solvers/linear/linear_solver_internal.h	Exposes new OMP factory prototypes under CFD_ENABLE_OPENMP.
lib/include/cfd/solvers/poisson_solver.h	Adds solver type string constants for the new OMP variants.
lib/CMakeLists.txt	Adds new OMP solver sources to the OpenMP build source list.
CHANGELOG.md	Documents the new OpenMP backends and their selection/consistency guarantees.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

🔵 Needs a closer look

Add 3D OpenMP coverage for the new Jacobi and BiCGSTAB paths.

Review details

Suppressed comments (3)

tests/math/test_omp_consistency.c:410

• These new consistency tests only initialize nz=1, but both implementations add a separate nz>1 path using k_start/k_end/stride_z. The existing 3D OMP suite covers only CG and Red-Black SOR (tests/math/test_poisson_3d.c:765-770), so z-plane indexing and boundary handling for the new Jacobi and BiCGSTAB backends can regress without being detected. Please add 3D OMP coverage for both methods, preferably comparing them with the scalar backends.

void test_jacobi_omp_vs_scalar(void) {

tests/math/test_omp_consistency.c:413

• This new consistency test only initializes a 2D problem (nz == 1), so it never exercises the new Jacobi OMP 3D k_start/k_end and stride_z path. The existing 3D OMP suite covers CG and Red-Black SOR but not Jacobi; add a 3D Jacobi OMP-vs-scalar or convergence case before relying on this backend for 3D projection solves.

void test_jacobi_omp_vs_scalar(void) {
    double dx = (XMAX - XMIN) / (NX - 1);
    double dy = (YMAX - YMIN) / (NY - 1);
    size_t n = NX * NY;

tests/math/test_omp_consistency.c:514

• This new consistency test also hard-codes nz == 1, leaving the new BiCGSTAB OMP 3D stencil and per-plane parallel primitives untested. The existing 3D OMP tests register only CG and Red-Black SOR, so add a 3D BiCGSTAB OMP-vs-scalar or convergence case to cover the path used by 3D callers.

void test_bicgstab_omp_vs_scalar(void) {
    double dx = (XMAX - XMIN) / (NX - 1);
    double dy = (YMAX - YMIN) / (NY - 1);
    size_t n = NX * NY;

• Files reviewed: 11/11 changed files
• Comments generated: 0 new
• Review effort level: Lite