compare-cpython-versions

# Compare CPython versions Use the [CPython dependency inventory](../find-cpython-usage/SKILL.md) to scope this work. Resolve exact old/new tags or commits; distinguish a release tag from a moving development branch. Use an existing CPython checkout when available, or a separate checkout of `https://github.com/python/cpython`. Do not switch the user's working checkout to inspect another ref. Run in the CPython checkout, substituting verified refs and paths: ```bash git diff <old-ref> <new-ref> -- <relevant-header-or-source> git log <old-ref>..<new-ref> -- <relevant-header-or-source> git log -p <old-ref>..<new-ref> -S '<changed-symbol>' -- Include Python Objects Modules ``` Use the inventory to check these change categories; this is not an exhaustive compatibility checklist: - Struct fields added, removed, renamed, reordered, or retyped, including pointer-to-tagged-reference changes, bitfields, alignment, and offsets used by copied layouts. - Headers or declarations moved, split, or merged; changed include order, guards, and build macros. - Functions or macros added, removed, or deprecated; changed signatures, replacements, and contracts. For local examples, inspect `ddtrace/internal/datadog/profiling/profiling_helpers/version_compat.h` and `frame_accessors.h` in that directory: they handle moved interpreter-frame headers and different representations of the frame's executable field. Verify each comparison against the selected versions. Inspect implementations as well as declarations: layouts, ownership, GIL requirements, finalization, and error behavior can change without a signature change. Use the exact revision's source, What's New notes, and linked upstream issues to explain each relevant difference. Follow `gh-<number>` or issue references in introducing commits to the CPython issue/PR and any linked PEP. `ddtrace/internal/datadog/profiling/stack/echion/echion/cpython/tasks.h` cites `gh-123923` for stack references; a proposal or release-note summary does not replace source verification. Connect each changed field or API to ddtrace's reads, macros, offsets, copies, and callers. Record the old/new contract, replacement or adaptation, required version guard, and affected files; include caller and upstream source links so the proposed changes can be checked. Report affected ddtrace callers, the changed contract, required adaptation, and validation across the supported versions/platforms. Treat pre-release behavior as version-specific; do not turn a snapshot into an undated universal rule.