feat: support remapping huge pages and freeing subtrees

[zhangyx827]

• Support PageSize::Size1G
  • Locate the P3 entry (handles M::LEVELS == 3/4), replace it with a huge-page PTE via GenericPTE::new_page(pa, flags,
    true)
  • Save the previous lower-level table address and free the old P2 subtree with dealloc_tree(p2table_paddr, M::LEVELS - 2)
• Support PageSize::Size2M
  • Walk to the P2 table through the P3 entry, replace the target P2 entry with a huge-page PTE
  • Free the old P1 subtree with dealloc_tree(p1table_paddr, M::LEVELS - 1)
• Reject non-huge page sizes: log an error and return PagingError::NotAligned
• Conservatively set self.flush = ToFlush::Full since a whole huge-page range is modified