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

docs/src/SUMMARY.md

@@ -19,7 +19,7 @@
 - [Python as Source of Truth](./architecture/python-source-of-truth.md)
 - [Silo Server](./architecture/silo-server.md)
 - [Signal Architecture](./architecture/signal-architecture.md)
-- [OndselSolver](./architecture/ondsel-solver.md)
+- [KCSolve: Pluggable Solver](./architecture/ondsel-solver.md)
 
 # Development
 
@@ -64,3 +64,4 @@
 - [OriginSelectorWidget](./reference/cpp-origin-selector-widget.md)
 - [FileOriginPython Bridge](./reference/cpp-file-origin-python.md)
 - [Creating a Custom Origin (C++)](./reference/cpp-custom-origin-guide.md)
+- [KCSolve Python API](./reference/kcsolve-python.md)