Release provenance: cosign checksum signing, SBOM, SLSA attestation

[robbiet480]

Releases publish checksums.txt and nothing else. There is no way for a consumer to verify that a binary came from this repository's pipeline.

Wanted

• Sign the checksum file with cosign (keyless / OIDC)
• Publish an SBOM per artifact
• Emit SLSA build provenance attestation

Notes: relevant to the supply-chain expectations under NIST 800-171 Rev. 3; verification steps belong in the README once shipped.

Priority: P3