Add native STRUMPACK sparse solver extension

[AJ0070]

This PR adds native STRUMPACK support to LinearSolve via a package extension.

What changed

• Added new sparse algorithm type: STRUMPACKFactorization in src/factorization.jl
• Exported and wired algorithm selection in src/LinearSolve.jl
• Added extension implementation in ext/LinearSolveSTRUMPACKExt.jl:
  • runtime discovery of libstrumpack
  • sparse CSC (AbstractSparseMatrixCSC{<:AbstractFloat}) support
  • CSC → CSR conversion for STRUMPACK C API
  • factorization/solve flow with SciML retcode mapping
  • cache + finalizer-based resource cleanup
• Registered extension in Project.toml
• Added/updated test coverage:
  • availability behavior in test/defaults_loading.jl
  • integration via generic resolve path in test/resolve.jl
  • optional dedicated testset wiring in test/runtests.jl (kept, but not required for CI coverage)

CI behavior

Current CI already runs STRUMPACK-related coverage through existing groups (Core, DefaultsLoading).
No CI workflow update is required unless we want the dedicated LinearSolveSTRUMPACK group to run explicitly.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

• STRUMPACK is loaded dynamically; if unavailable, constructor/solve paths fail with clear user-facing messages.
• The implementation currently targets STRUMPACK MT sparse interface and Float64 solve path.

AI disclosure: used codex and gemini to help understanding current state of the code with respect to issue, adding docstring, refactoring code, adding comments and refactoring documentation, formatting PR description.

closes #170