Function map_page_in_space 

fn map_page_in_space(
    frame_phys: u64,
    target_va: u64,
    address_space: &AddressSpace,
) -> Result<(), &'static str>

Map a physical page into an address space at the specified virtual address.

The mapping is kernel-only (no USER_ACCESSIBLE flag) to enforce the spec §8.4 invariant: frame permissions are managed exclusively by the kernel, never by non-kernel parties.