🐛 Initialize the Qiskit C API once

[simon1hofmann]

🤖 AI text below 🤖

Description

Initialize the vendored Qiskit C API once per versioned translation unit through thread-safe function-local static initialization. This publishes the capsule-backed function table safely when nanobind 3 free-threaded callers create translations concurrently.

Add a focused concurrent import/export regression. This is a prerequisite for #2176 and follows the split-mode/free-threaded binding adoption in #2209.

Testing

• The release split-mode binding containing this initialization hunk built successfully.
• The concurrent regression passed as part of all 218 Qiskit translation tests on the stacked branch.
• Clang formatting, Ruff, and git diff checks passed for the isolated patch.

AI assistance: Codex assisted with implementation review, isolation, validation, and this description.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[burgholzer]

LGTM 👍🏼
The PR is still missing tags and a completed checklist. Then this can go in directly 😌