ENH: Serialize null interface refs as literals

[aminemanai2003]

Summary

• serialize deleted modelx interface references as literal Interface tuples instead of storing their null objects in data.pickle
• restore the original null-interface type without relying on binary data
• add regression coverage for serializer versions 6, 7, and 8

Root cause

InterfaceRefEncoder only accepted valid interfaces. A reference to deleted Cells or Spaces therefore fell through to PickleEncoder, producing an opaque entry such as ("Pickle", 1) in the generated source.

Null interfaces now use an explicit representation such as ("Interface", (None, "Cells")). The outer tuple remains a two-element interface reference, retaining the existing automatic-reference restoration behavior when a deleted target can no longer satisfy a relative reference.

Validation

• python -m pytest modelx/tests/serialize -q — 250 passed, 2 skipped
• python -m pytest modelx/tests -q — 1221 passed, 6 skipped
• git diff --check

Closes #226