Please ignore this PR until reviewed by @ChrisRackauckas.

Summary

• restore cross-process serialization for RuntimeGeneratedFunctions after drop_expr
• serialize dropped functions through an internal proxy that carries the cached body, using only the public Serialization.serialize/deserialize API
• keep the deserialized function dropped while restoring its destination-process cache
• restore the cross-process regression test for both retained and dropped functions

Root cause

#136 removed the dropped-function serializer while migrating strict QA because it called the non-public Serialization.serialize_type. #137 then released that behavioral break as v0.5.23. ModelingToolkitBase has intentionally dropped generated-function bodies since April 2025, so its clean-master distributed test began failing when the worker deserialized an RGF produced in another process. The first bad source commit is 6550973d9290c906b1f61d37d416695e95e63a7c; v0.5.22 is the passing dependency control.

This implementation avoids the non-public API that motivated the removal.

Validation

• julia +1.10 --project=. -e 'using Pkg; Pkg.test()'
• julia +1.12 --project=. -e 'using Pkg; Pkg.test()'
• Runic 1.7.0: --check --diff .
• ModelingToolkitBase clean master d39174937fead779b29fa5baa50ba975adbde8c9, Julia 1.10, official GROUP=Extended:
  • RuntimeGeneratedFunctions v0.5.23: reproduces ArgumentError: cannot deserialize a dropped RuntimeGeneratedFunction
  • pinned v0.5.22: passes, including Distributed Test
  • this branch developed locally as v0.5.23: passes, including Distributed Test and the full Extended group

Downstream failure: SciML/ModelingToolkit.jl#4852.