release: enforce immutable published release assets

[codeforester]

Goal

Ensure a published base-cli release cannot have its artifact bytes silently replaced after publication.

Background

The package workflow currently uploads release assets with gh release upload --clobber. That can make a rerun convenient, but it conflicts with the immutable published-release policy used by base-demo and with the provenance model required for reproducible ecosystem releases. An existing release should either be proven identical or require a new version.

Scope

• Remove unconditional --clobber behavior for published release assets.
• Fail closed when an existing asset has different bytes, filename identity, or checksum.
• Allow an idempotent rerun only when the existing asset is byte-for-byte identical and the release/tag identity matches.
• Require a new patch version for corrections to published assets.
• Update release documentation, workflow tests, and the cross-repository release policy.
• Preserve trusted publishing, SBOM, checksum, and attestation behavior.

Acceptance Criteria

• A release job cannot overwrite a different existing asset.
• A rerun with identical assets succeeds without changing release history.
• A mismatched asset produces a clear failure with the expected and observed checksums.
• A tag, version, release, asset, SBOM, and provenance identity are checked together.
• Negative fixtures cover changed bytes, changed filenames, changed tag target, and rerun behavior.
• Release documentation states the correction path as a new patch release.

Validation

• Run the package workflow contract and release metadata tests.
• Rehearse first-publication, identical-rerun, and mismatched-rerun cases in a temporary release fixture.
• Verify the resulting behavior matches base-demo and the ecosystem release BOM policy.
• git diff --check.

Non-Goals

• Do not replace trusted PyPI publishing or artifact attestations.
• Do not block a safe pre-publication build retry that has not created a public release.
• Do not alter already-published historical assets.

Project Fields

• Status: Backlog
• Priority: P1
• Size: M
• Area: Packaging
• Initiative: v1.0 Readiness

Ownership

• Assignee: @codeforester

Related: #271, #307, and basefoundry/base#2115.