Track runtime enforcement: Lock-Chain Consistency

[mdheller]

Goal

Track runtime enforcement of append-only, chained source locks for ArtifactSourceLock.

Layer 1 schema enforces presence, frozen metadata, and content-hash. Layer 2 must enforce chain integrity: no overwrite, consistent hash propagation, append-only policy.

Acceptance Criteria

• Attempted overwrite of a previous lock triggers runtime failure.
• Any chain break between artifact versions is detected.
• CI fixture placeholder demonstrates the intended runtime failure if violated.

This issue provides a placeholder to reference in PR #27 for Layer 2 enforcement (#N).