Add libneo sparse_mod integration test consolidating NEO-2/MEPHIT/KAMEL solver

[slopqueue]

Closes #13

Problem

NEO-2, MEPHIT and KAMEL each carried a byte-level fork of sparse_mod, the
UMFPACK/SuiteSparse front end used for their linear systems. Maintaining
three copies is error-prone; issue #13 asks for a single libneo "unique version"
that all codes link against.

Change

This PR adds tests/libneo/src/sparse/test_sparse_mod.f90, an integration test
for the consolidated libneo sparse_mod shipping as the LIBNEO::sparse
target. It exercises the API surface the three consumers rely on:

• (a) the sparse_example(1) reference vector kept in
  KAMEL/QL-Balance/src/test/test_sparse.f90
• (b) the decoupled-DOF case (structurally empty row/column), which must now
  return exact zeros instead of uninitialised heap
• (c) the complex overload
• (d) sparse_solve_method 2 vs 3 (umf4solr vs umf4sol) both converge
• (e) the iopt 1/2/3 factorize-solve-free cycle used by
  KAMEL/QL-Balance/src/base/evolvestep.f90

The subdirectory is registered in tests/libneo/src/CMakeLists.txt only when
the installed libneo provides the optional sparse target, so hosts without a
SuiteSparse build are unaffected.

Tests

Regression coverage runs as the test_sparse_mod CTest (LABELS sparse)
under tests/libneo, invoking the consolidated libneo solver and failing
with error stop on any residual or reference mismatch.

[krystophny]

Review verdict: Request changes

Summary: The PR adds a useful sparse solver integration test, but its CMake guard prevents the test from ever being registered in the standalone tests/libneo project.

Findings:

1. [major] tests/libneo/src/CMakeLists.txt:22 — This project never calls find_package(libneo) or adds libneo’s CMake targets, so TARGET sparse is false even when the installed libneo provides LIBNEO::sparse. A local configure registers only the existing field test, and CI does not run this test directory. Import/discover the libneo target correctly or use an appropriate availability check, and ensure CI executes the resulting CTest.

Verdict: Request changes — the added regression test is currently silently skipped.

[krystophny]

Review verdict: Request changes

Summary: The PR adds useful sparse solver coverage, but the standalone CMake project does not import libneo’s LIBNEO::sparse target or its SuiteSparse dependencies.

Findings:

1. [major] tests/libneo/src/sparse/CMakeLists.txt:4 — sparse is linked as a bare -lsparse, while this project defines no sparse target. For libneo’s static sparse archive, its transitive UMFPACK/SuiteSparse dependencies are therefore omitted and the test will fail to link when the archive is found. Import/link the exported LIBNEO::sparse target (including its module path), or explicitly provide the complete dependency closure.

Verdict: Request changes — the test is not buildable on hosts where the sparse library is available.

[krystophny]

Review verdict: Request changes

Summary: The sparse integration test provides useful coverage, but it is not executed by the repository’s CI workflow.

Findings:

1. [major] tests/libneo/src/CMakeLists.txt:24-26 — The test is registered only in the standalone tests/libneo CMake project, which .github/workflows/main.yml never configures or runs. CI run #349 succeeds without compiling or executing test_sparse_mod, so regressions in this new coverage can pass unnoticed. Add this project to CI and run its CTest suite.

Verdict: Request changes — the added regression test is currently unverified by CI.