Improve model supply chain retention evidence gates

[danyili2632]

Summary

• addresses [REVIEW] model-supply-chain: add yanked-model and registry-retention evidence gates #1331 by adding registry availability and rollback continuity coverage to model-supply-chain
• adds evidence gates for immutable internal mirrors, retention duration, complete artifact sets, license/access snapshots, rollback copies, and rebuild/rollback drills
• updates the assessment output template and maturity summary so reviewers capture these continuity risks explicitly

Validation

• git diff --check
• verified required content markers for registry continuity, internal mirrors, artifact retention, adapter/base-model identity, license snapshots, and rollback drill evidence
• verified Markdown fence count is balanced

Bounty

• Target tier: Improver Moderate ($100) if accepted
• Preferred payment method: crypto, Base USDC 0x6CBF4b5cb88b8C2B7af776Bc2B073163B5d3C08A