Function create_user_test_task 

pub fn create_user_test_task()

Create and schedule a minimal Ring 3 test task.

This function:

1. Creates a user address space (PML4 with kernel half cloned)
2. Maps a code page at USER_CODE_ADDR (RX, user-accessible)
3. Maps a stack page at USER_STACK_ADDR (RW, user-accessible)
4. Writes a small machine code blob into the code page
5. Creates a kernel task whose entry point does IRETQ to Ring 3