Self-host build backend for offline readiness

[SaridakisStamatisChristos]

Summary

• replace the external setuptools build backend with an in-tree backend that vendors all metadata and emits wheels/sdists using only the standard library
• document the production-ready offline workflow, supported platform matrix, and add a signed release checklist to guide artifact verification
• expand CI with a cross-platform matrix plus an offline install smoke test to keep the no-index workflow exercising on every change

Testing

• python -m pip install --no-index .

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https://chatgpt.com/codex/tasks/task_e_68e62655a3b88333948795c077496612