refactor: address review feedback

Refactor i686 page table code to follow the same architecture-independent
API as amd64/aarch64, per reviewer feedback.

hyperlight_common changes:
- Extract shared page table iterators (modify_ptes, read_pte_if_present,
  require_pte_exist, write_entry_updating) to vmem.rs with PTE_SHIFT
  const generic for reuse across architectures.
- Implement i686/vmem.rs with proper map()/virt_to_phys() using the
  shared iterators, replacing stubs and the host-side i686_guest
  submodule that was in amd64/vmem.rs.
- Remove PAGE_USER from pte_for_table (PDEs are supervisor-only by
  default); user-space PDEs get PAGE_USER via post-processing.
- Remove SCRATCH_TOP_PD_ROOTS_* and MAX_PD_ROOTS constants (replaced
  by set_pt_root_finder callback).

hyperlight_host changes:
- Unify GuestPageTableBuffer<PTE_BYTES> for both architectures,
  removing i686_pt::Builder, build_initial_i686_page_tables, and
  compact_i686_snapshot (~500 lines removed from snapshot.rs).
- Add root_offset to GuestPageTableBuffer for targeting per-process
  PDs, with finalize_multi_root() to replicate kernel PDEs, map
  scratch, and set PAGE_USER across all roots.
- Snapshot::new uses filtered_mappings with root-index tagging for
  per-process PD isolation in a single pass (no double PT walk).
- Add set_pt_root_finder callback on MultiUseSandbox, replacing the
  scratch-based PD roots bookkeeping.
- Add CR0 named constants (CR0_PE, CR0_ET, CR0_WP, CR0_PG).
- Add compaction_kind() helper to deduplicate kind conversion.
- Add 5 i686 unit tests for map/virt_to_phys.
- Detailed comments on separate_pt_bytes explaining the map_file_cow
  GPA overlap constraint.

Author: ludfjig

src/hyperlight_common/src/arch/aarch64/vmem.rs

@@ -44,9 +44,17 @@ pub unsafe fn virt_to_phys<'a, Op: TableReadOps + 'a>(
     core::iter::empty()
 }
 
-pub trait TableMovability<Op: TableReadOps + ?Sized, TableMoveInfo> {}
-impl<Op: TableOps<TableMovability = crate::vmem::MayMoveTable>> TableMovability<Op, Op::TableAddr>
+impl<Op: TableOps<TableMovability = crate::vmem::MayMoveTable>> crate::vmem::TableMovability<Op>
     for crate::vmem::MayMoveTable
 {
+    type RootUpdateParent = crate::vmem::UpdateParentRoot;
+    fn root_update_parent() -> Self::RootUpdateParent {
+        crate::vmem::UpdateParentRoot {}
+    }
+}
+impl<Op: TableReadOps> crate::vmem::TableMovability<Op> for crate::vmem::MayNotMoveTable {
+    type RootUpdateParent = crate::vmem::UpdateParentNone;
+    fn root_update_parent() -> Self::RootUpdateParent {
+        crate::vmem::UpdateParentNone {}
+    }
 }
-impl<Op: TableReadOps> TableMovability<Op, Void> for crate::vmem::MayNotMoveTable {}