Prepare stable 1.0.0 release

[eigger]

Summary

• Prepare package metadata, README, and changelog for stable 1.0.0.
• Advance the generated payload schema version to 3 for the post-0.5 contract additions.
• Verify release tags match the project version before publishing.

Verification

• Sol 6.1 release review: no remaining findings.
• 1725 tests passed locally.
• Ruff, mypy, and schema export checks passed.
• Built sdist and wheel; twine check dist/* passed.
• Verified v1.0.0 matches the package metadata.

This prepares the release commit only; it does not create a tag or publish to PyPI.