Split py_nif.c into logical modules for better maintainability

Refactor the monolithic 4,335-line py_nif.c into organized modules:
- py_nif.h: Shared header with types, macros, and declarations
- py_convert.c: Type conversion between Python and Erlang
- py_exec.c: Python execution, timeout support, executor threads
- py_callback.c: Erlang callback module, suspension state, asyncio

Uses #include approach for single compilation unit, avoiding CMake
changes while organizing code logically.

Author: benoitc