Refactor DeltaSpin PW subspace cache with std::vector

[hujieting]

Summary

This PR refactors the CPU memory management of SubspaceCache in source/source_lcao/module_deltaspin/deltaspin_pw_cache.h, replacing manually managed host buffers with std::vector<std::complex<double>> while preserving existing interfaces.

Changes

• Replace three CPU raw-pointer buffers with std::vector<std::complex<double>>.
• Use resize() for CPU allocation and empty-vector swap() for storage release.
• Preserve raw-pointer and per-k-point accessors.
• Separate GPU device pointers from CPU vector storage while retaining existing CUDA/ROCm allocation operators.
• Leave the lambda snapshot unchanged.

Validation

• git diff --check: passed.
• MPI/LCAO incremental build: passed.
• DeltaSpin unit tests: 6/6 passed.

Limitations

• CUDA/ROCm build and runtime paths have not been tested.
• Static review identified potential edge cases in allocation-state tracking, empty-cache access, and exceptional-path GPU cleanup. No confirmed failure was found in the reviewed normal execution paths.

[mohanchen]

This is a bit complicated, I will close it for now.