DEN-3908: sign TJSV contract evidence and bind it to package releases #74

Add release-level provenance that binds published Zed packages to the exact admitted contract evidence.

Acceptance:

• package/release provenance includes TJSV revision, receipt schema, irId, run id, source digests and generated-output digests;
• sign/attest the evidence with an approved CI identity;
• verification runs before registry publication and on install/verification paths where applicable;
• reject unsigned, expired, stale, mismatched, or tombstoned contract evidence;
• distinguish hash binding from publisher authenticity;
• preserve reproducible unsigned local development without mislabeling it release-certified;
• sibling e2e test tampers receipt/signature/output independently and proves rejection.

Linear parent/related: DEN-3908, DEN-52, DEN-3828.