docs: KCSolve architecture and Python API reference
- Replace OndselSolver architecture doc with KCSolve pluggable solver
architecture covering IKCSolver interface, SolverRegistry, OndselAdapter,
Python bindings, file layout, and testing
- Add kcsolve Python API reference with full type documentation, module
functions, usage examples, and pybind11 vector-copy caveat
- Add INTER_SOLVER.md spec (previously untracked) with Phase 1 and Phase 2
marked as complete
- Update SUMMARY.md with new page links