Add canonical cross-interpreter serialization tests and fixture bridges - #9408

Summary

Split the canonical cross-interpreter serialization test cohort out of #8954:

• Import the complete test_crossinterp module from CPython d220409a13b77de7a6c30abe16ea90db01528edc using scripts/update_lib migrate, without changed expectations or new markers.
• Add the two _testinternalcapi capsule bridges consumed by that module, reusing the existing cross-interpreter serialization and validation implementation.
• Keep pickle/marshal/script fixture modes in a private CrossinterpData wrapper instead of extending the production SharedValue representation or public API.

The supporting _code_definitions and _crossinterp_definitions modules already match the same CPython revision. No lifecycle, interpreter creation/destruction, C API exports, parser or vendored dependency changes are included.

This is test infrastructure and canonical coverage, not a new production cross-interpreter API. Static source inventory finds 102 discoverable test methods (93 definitions plus inherited tests); this is not an execution result.

CPython comparison and simplification

Compared directly with CPython 3.15.0 (d220409a), including Modules/_testinternalcapi.c and Python/crossinterp.c. Follow-up 90bb550c2d52a156627ae391ca8a48fdfdcafb14 consolidates six constructor entry points into one mode dispatcher (19 fewer Rust lines), retaining the distinction between generic marshal and code transport, capsule ownership checks, and pure-script validation order. The canonical tests are unchanged; the four adjacent exception-capture tests cover existing behavior rather than justify new bridge code.

Validation

• Canonical module and supporting definitions: byte-for-byte comparisons passed.
• Normal check-merge-conflict, Ruff format/check, redundant-test-patches and rustfmt hooks passed; prettier had no applicable files. git diff --check passed. Only cspell was omitted locally with explicit user authorization. Lint CI, including cspell, passed for simplified head 90bb550c2d52a156627ae391ca8a48fdfdcafb14.
• Focused default VM library cargo check: passed on the simplified source.
• Strict default VM library Clippy: fails at unchanged _ctypes/base.rs:2246, arc_with_non_send_sync on Arc<BufferWrite> in non-threading mode. The entire file matches main df0d1b74; a separate baseline Clippy run was not performed. No lint suppression or unrelated ctypes changes are included.
• Focused threading VM library check and strict Clippy: passed on the simplified source; bound input closure verified before and after.
• Local canonical runtime module, full interpreter build and aggregate repository test suites were not run. Linux, macOS and Windows CI passed their interpreter builds, snippets and canonical suites, with test_crossinterp passed explicitly recorded on all three for simplified head 90bb550c2d52a156627ae391ca8a48fdfdcafb14. macOS reached its final success after the test runner handled an environment-changed result. Lib dependency checks also passed. The complete 24-job correctness workflow and Lib dependency workflow passed for this exact simplified head. This PR remains a draft for review.

The focused checks are source/cache/toolchain-bound, offline and resource-bounded. Independent review covered capsule ownership, private fixture boundaries and script-validation ordering. Local runtime testing remains separate: the canonical module creates a temporary import fixture and reads supporting definition files, so it is not a read-only test.