## Why
The SDK release workflow published the runtime before building the SDK, so an SDK build failure could leave the runtime published on its own.
## What changed
- Extract a reusable SDK build workflow that packages checked-in generated code and runs alongside runtime preparation. Require both builds before publishing the runtime, and verify runtime availability before publishing the SDK.
- Add `stage-sdk --codex-version` to set an explicit runtime dependency independently of the SDK version, retaining the checked-in pin by default and rejecting missing or duplicate pins.
- Accept Codex release tags in the runtime version resolver and use its normalized Python version for standalone runtime PyPI verification.
## Testing
Add coverage for wheel and source distribution metadata, preservation of checked-in code, independent beta SDK versions, runtime tag normalization, and invalid versions or dependency pins.
GitOrigin-RevId: feb572fadcf5d148814b26b480b4bae5ef6a39c5