* temp regrtest save_env patch
* Refactor signal_handlers from Option to OnceCell
- Change `signal_handlers` type from `Option<Box<...>>` to
`OnceCell<Box<...>>` so fork children can lazily initialize it
- Add `VirtualMachine::is_main_thread()` using runtime thread ID
comparison instead of structural `signal_handlers.is_none()` check
- Initialize signal handlers in `py_os_after_fork_child()` via
`get_or_init()` for workers that fork
- Use `get_or_init()` in `signal()` and `getsignal()` functions
- Remove `set_wakeup_fd(-1)` special-case workaround (d6d0303)
- Extract `signal::new_signal_handlers()` to deduplicate init expr
- Allow `getsignal()` from any thread (matches CPython behavior)
- Fix `set_wakeup_fd` error message to name the correct function
