Function validate_user_region 

fn validate_user_region(
    base: u64,
    len: usize,
    access: Access,
) -> Result<(), UserSliceError>

Validate that a user memory region [base, base+len) is:

• entirely within the user address space
• mapped with the required permissions in the active page tables

Returns Ok(()) on success, or a UserSliceError describing the problem.