Attest release distribution provenance

[biru-codeastromer]

Summary

• add pinned Sigstore build-provenance attestations for the exact wheel and source archive subjects listed in SHA256SUMS
• inherit Publish assets before immutable release lock #35 protected publication and Verify release assets before upload #24 exact builder, checksum, draft recovery, and remote-asset verification
• verify every distribution attestation against the exact release workflow, workflow revision, protected tag ref, and source commit before closure
• keep GitHub automatic release-attestation verification as a separate publication control

Validation

• make release-gate: 111 passed, 1 optional skip, 91.83% coverage
• committed workflow tests assert pinned attestation, exact subjects, signer workflow and digest, protected source ref and digest, exact remote assets, and immutable closure
• package, dependency, example, text, repository, and Gitleaks audits passed

Integration order is #35, then #24, then this PR. This PR still requires an eligible reviewer approval. No release or tag was created.

[biru-codeastromer]

The provenance-documentation blocker is fixed at the new head. README now states that v0.2.3 predates the attestation workflow and has no distribution attestations, and uses a version-neutral X.Y.Z verification example for future workflow-produced releases. The corrected #24 checksum scope is also carried forward. Fresh local gate: 101 passed, 1 optional skip, 91.83% coverage, clean installs and all audits green.

[biru-codeastromer]

Superseded by #37, which preserves the provenance contract on a clean linear history and verifies exact artifact identity before publication.