Tupek/tr core mfem subspace

[tupek2]

• Remove PETSc/SLEPc dependency from trust-region subspace solver.
• Replace PETSc reduced solve with MFEM/LAPACK implementation.
• Cache reduced subspace projections across trust-radius retries.
• Extract Steihaug-Toint CG into private implementation helper.
• Keep public entry through NonlinearSolver::TrustRegion.
• Use one batched global dot-product path for trust-region math.
• Add previous accepted steps to subspace basis, default 2.
• Add tuned trust-region defaults from benchmark runs.
• Add MFEM subspace and Steihaug-Toint CG tests.
• Guard LAPACK subspace tests on MFEM_USE_LAPACK.
• Add shallow-arch snap-through physics coverage.
• Add Inlet parsing for NewtonLineSearch and TrustRegion.