* Add canonical cross-interpreter serialization tests and fixture bridges
Import the complete CPython test_crossinterp module and provide its capsule bridges using existing serialization and validation logic. Keep fixture modes private instead of expanding SharedValue or interpreter lifecycle APIs.
Assisted-by: OpenAI Codex: model version unavailable
* Simplify canonical cross-interpreter fixture dispatch
Match CPython 3.15.0 mode dispatch with one private constructor instead of six forwarding entry points. Keep generic marshal data distinct from code transport, preserve capsule ownership checks and pure-script validation, and leave the canonical tests unchanged.
Assisted-by: OpenAI Codex: model version unavailable
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Co-authored-by: youknowdot <336763669+youknowdot@users.noreply.github.com>
