Change impact analysis via artifact hashing

[avrabe]

Context

Eclipse SCORE needs change impact analysis (docs-as-code #314, process_description #535) to answer: "if I change this requirement, what downstream artifacts are affected?" They're waiting on upstream sphinx-needs to implement hash-based versioned links.

Problem

When an artifact changes (title, description, fields, links), all downstream artifacts that depend on it may need review. Currently there's no automated way to:

1. Detect which artifacts changed between two baselines/commits
2. Compute the transitive impact set via the link graph
3. Flag downstream artifacts as "needs review"

Rivet already has rivet diff for structural comparison and petgraph for reachability — combining them solves this.

Proposed approach

1. Content hashing: compute a stable hash of each artifact's content (title + description + fields + links)
2. Stored hashes: optional rivet.lock or per-artifact hash field for baseline comparison
3. Impact command: rivet impact --since <commit|tag> that:
   • Diffs current artifacts against the baseline
   • For each changed artifact, walks the link graph to find all transitively dependent artifacts
   • Reports the impact set grouped by depth (direct dependents, transitive)
4. Dashboard integration: highlight impacted artifacts in the graph/matrix views

Rivet artifacts

• REQ-024
• FEAT-041

References

• SCORE issue: eclipse-score/docs-as-code#314
• SCORE issue: eclipse-score/process_description#535

[avrabe]

Implemented in PR #27 (feat/phase3-integration)