Add the kcsolve pybind11 module exposing the KCSolve C++ API to Python:
- PyIKCSolver trampoline enabling Python IKCSolver subclasses
- Bindings for all 5 enums, 10 structs, IKCSolver, and OndselAdapter
- Module functions wrapping SolverRegistry (available, load, joints_for,
set_default, get_default, register_solver)
- PySolverHolder class forwarding virtual calls with GIL acquisition
- register_solver() for runtime Python solver registration
IKCSolver constructor moved from protected to public for pybind11
trampoline access (class remains abstract via 3 pure virtuals).
Includes 16 Python tests covering module import, type bindings, enum
values, registry functions, Python solver subclassing, and full
register/load/solve round-trip.
Closes#288
