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+# ADR-CE-003: PostgreSQL + Ruvector Unified Substrate
+
+**Status**: Accepted
+**Date**: 2026-01-22
+**Parent**: ADR-014 Coherence Engine Architecture
+
+## Context
+
+The coherence engine requires:
+- Transactional authority for governance data (policies, witnesses, lineage)
+- High-performance vector/graph operations for coherence computation
+- Audit trail with deterministic replay
+
+## Decision
+
+**PostgreSQL + ruvector as unified substrate.**
+
+| Layer | Storage | Purpose |
+|-------|---------|---------|
+| Governance | PostgreSQL | Policy bundles, witnesses, lineage (ACID) |
+| Coherence | ruvector | Node states, edges, HNSW index, residuals |
+| Audit | PostgreSQL | Event log with signatures |
+
+## Consequences
+
+### Benefits
+- PostgreSQL: Battle-tested ACID for governance
+- ruvector: Optimized for vector similarity and graph traversal
+- Clear separation of concerns
+
+### Risks
+- Two systems to maintain
+- Cross-system consistency requires careful transaction handling
+
+## References
+
+- ADR-014: Coherence Engine Architecture, Section 13