Add OWN_GIL mode for true parallel Python execution

Each OWN_GIL context gets a dedicated pthread with its own GIL,
enabling true parallel CPU-bound execution (4x speedup with 4 cores).

- Extend py_context_t with OWN_GIL fields
- Implement owngil_context_thread_main() and dispatch_to_owngil_thread()
- Register erlang module in OWN_GIL subinterpreters
- Add owngil mode to py_context.erl
- Add test suite and benchmark

Requires Python 3.12+.

Author: benoitc