COMP: Apply C++ Core Guidelines smart pointer fixes

[hjmjohnson]

Summary

Replace raw pointer ownership patterns with std::unique_ptr across three modules, following C++ Core Guidelines R.11 (avoid calling new and delete explicitly) and R.20 (use unique_ptr to represent ownership).

• FEMLinearSystemWrapperDenseVNL: Replace raw new/delete of matrix and vector holders with std::unique_ptr and std::make_unique. Use std::swap for all swap methods. Default the destructor.
• CellInterfaceTest: Replace raw new[]/delete[] array allocation with std::vector, eliminating manual memory management in test code.
• VoxBoCUBImageIO: Replace raw pointer return types and members with std::unique_ptr<GenericCUBFileAdaptor>, using std::make_unique for polymorphic factory creation. Default the destructor.

OpenCVVideoIO changes split to separate PR: #5994

Test plan

• Verify CI builds pass on all platforms (Linux, macOS, Windows)
• Verify existing CTest tests pass (no behavioral changes, only ownership semantics)
• Verify FEM module tests pass
• Verify Mesh/CellInterface test passes
• Verify VoxBoCUB IO tests pass

🤖 Generated with Claude Code

[hjmjohnson]

All issues from the greptile review have been addressed in the force-pushed v3:

• P0 – Orphaned code at global scope: OpenCVVideoIO changes split out to PR COMP: Migrate OpenCV Video Bridge from legacy C API to C++ API #5994, which was written clean without this bug.
• P0 – catch (..) typo: Fixed to catch (...) with three dots.
• P1 – Writer not closed after Write(): m_Writer.reset() now called at end of Write(), preserving original flush/close behavior.
• P2 – SwapVectors inconsistency: Now uses std::swap on the unique_ptrs, consistent with SwapMatrices and SwapSolutions.

[dzenanz]

Looks good on a glance. I would prefer is someone else reviewed this too.

[N-Dekker]

In the past, the use of {} appeared problematic for smart pointers to a forward-declared class:

• COMP: Remove in-class {} member initializers of unique_ptr #3877

I don't know if that is still relevant nowadays. 🤷 For GCC std::unique_ptr, it was fixed with GCC 9.2. Do we require GCC >= 9.2?

If the {} do still cause a problem, simply remove them. Default-initialization of std::unique_ptr will work fine anyway!

[dzenanz]

If memory serves me well, we support a rather old GCC version.

[N-Dekker]

GCC 7, according to:

ITK/Documentation/docs/supported_compilers.md

Line 27 in aed0427

* GCC 7 and later [should be supported](https://www.gnu.org/software/gcc/projects/cxx-status.html).

So unfortunately then, these particular {} should be removed!

(The {} problem is specific for smart pointers to a forward-declared class. In other cases, {} usually just works fine.)

[hjmjohnson]

Working on it now.

[N-Dekker]

Sorry, but again, I would prefer these commits to be in separate PRs.

[hjmjohnson]

Closing in favor of three focused single-commit PRs:

• COMP: Use std::unique_ptr in FEMLinearSystemWrapperDenseVNL #5995 — COMP: Use std::unique_ptr in FEMLinearSystemWrapperDenseVNL
• COMP: Replace raw new[] with std::vector in CellInterfaceTest #5996 — COMP: Replace raw new[] with std::vector in CellInterfaceTest
• COMP: Replace raw pointers with std::unique_ptr in VoxBoCUBImageIO #5997 — COMP: Replace raw pointers with std::unique_ptr in VoxBoCUBImageIO (also addresses @N-Dekker's GCC 7 {} comment)

Each PR contains exactly one commit and can be reviewed and merged independently.