Add new solver stepping methods

[jc-macdonald]

Extract the new stepping methods added in PR #18 into a focused PR.

Scope

• New method implementations in core_solver.py (~469 lines of additions)
• Address inline review feedback:
  • Fix duplicate op argument (L613)
  • Use NDArray[...] consistently instead of np.ndarray (L672)
  • Remove unnecessary static method indirection for jacobian(t, y) (L754)
• Plumbing test updates in test_core_solver_plumbing.py

Context

Decomposed from PR #18 which mixed solver methods, accuracy tests, examples, and README changes. This issue covers only the core method additions.

Ref: PR #18 review by @TimothyWillard