[FEATURE] Sign and verify release artifacts

Summary

Strix release v1.6.1 publishes binary and wheel assets without checksum or signature assets

The shell installer downloads an archive over HTTPS and installs it without verifying its contents against publisher-controlled provenance

HTTPS protects transport but does not give users an independent way to verify that a downloaded file is the intended Strix artifact

Current trust chain

1. The installer chooses an asset from the GitHub release
2. HTTPS protects the download in transit
3. The installer extracts and replaces files without a separate authenticity check

The missing step is proof that the downloaded bytes are the artifact produced by the Strix release workflow

Relationship to checksum work

Pull request #1154 adds SHA256SUMS

Checksums detect corruption only when users can trust the checksum source

Publisher signing or keyless build provenance would authenticate the release independently from an unsigned checksum file

Requested release flow

1. Publish a checksum for every asset
2. Publish a Sigstore signature or signed provenance statement for every binary and wheel
3. Verify both before the installer extracts or replaces files
4. Keep the existing installation when verification is missing or fails

The release notes should also show a manual verification command for users who download assets directly