What to build

Optimize BDF and ImplicitRungeKutta to use sparse matrix factorizations when the Jacobian is stored sparsely.

Acceptance criteria

• Stiff solvers detect and utilize MatrixStorage::Sparse.
• Linear system solver in src/linalg supports sparse input.
• Performance verification on a large, sparse stiff system.

Blocked by

• feat(linalg): SIMD-accelerated internal matrix operations