chore: declare privacy release line

[abrichr]

Summary

• label openadapt-privacy Experimental and distinguish API versioning from clinical validation
• explicitly keep semantic release on the already-published 1.x line
• replace the obsolete pre-1.0 regression assertion with the actual release policy

Context

The security-fixed release is publicly available as 1.0.0. Python Semantic Release 10 defaults allow_zero_version to false, so the previous major_on_zero=false assertion did not preserve a 0.x line. Public package history is retained rather than deleting or rewriting tags.

Verification

• 2 passed in tests/test_release_config.py
• Ruff passes
• TOML parses with allow_zero_version=false and major_on_zero=true
• git diff --check passes

This PR must be squash-merged with its chore: title so it does not create a package release.