Patch release that freezes the contract hardening and CI fixes on main.
- Adds v1.2.1 package/version pins in project metadata and README install commands.
- Keeps manifest producer.version aligned with the package version used by the source tree.
- Includes the Python 3.10 CI fix for test metadata parsing.
- Verified by GitHub CI on Python 3.10, 3.11, and 3.12.