v4.1.0

Release v4.1.0

Fix memory leaks, double JSON serialization, and iterator safety

- Clear global handler maps (ubus_methods, ubus_event_cbs) in ubus destructor
  to prevent leaks on re-instantiation
- Fix uloop::task::add() to use iterator instead of operator[] for safety
- Add JSON serialization error checks in call_async() and send_event()
- Initialize exit_task with memset before each use
- Reset uloop_initialized flag in uloop::run() cleanup
- Switch main.cpp to std::unique_ptr for exception-safe ubus lifetime

All changes verified with Valgrind: 0 leaks, 0 errors in example code.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>