[algebraic-dev]

This PR tears down the libuv event loop when a program returns from main, once its running tasks have finished:

• every handle is closed.
• in-flight requests complete or are cancelled
• the runtime now uses its own uv_loop_t instead of uv_default_loop() so every handle on the loop belongs to it.
• lean_init_task_manager after a finalization starts a fresh loop and loop thread
• every libuv entry point fails with UV_ECANCELED while the loop is torn down

These changes fixes crashes at exit when a libuv callback ran during runtime shutdown.