[Refactor] Implement Policy-Based Math Backends (Eigen Reference)

[csparker247]

Description

Refactor ABFPlusPlus to decouple the algorithm from the Eigen library. The algorithm will now interact with a Backend object constrained by a C++20 Concept.

Tasks

• Define include/OpenABF/Backend/Concepts.hpp using C++20 template <typename T> concept MathBackend.
• Implement include/OpenABF/Backend/Eigen.hpp as the primary reference backend.
• Update ABFPlusPlus::Compute to utilize the backend for:
  • Sparse matrix assembly from std::span<Triplet>.
  • Linear algebra primitives (spmv, axpy, invert_diagonal).
  • Preserving Eigen's expression templates through proxy return types.
• Unit test: Ensure current ABF++ results are identical after the refactor.