feat: target-ontology-adapter §5 Materializer + graph storage

[deanban]

Neo4j materialization for model_role=TARGET nodes plus EnrichmentDecision storage. Includes GraphWriter protocol, idempotent MERGE keys, edge-endpoint constraint nodes, integrity check between compact Entity.enrichment_<facet>_status and structured EnrichmentDecision, and the new Cypher migration.

Tracking: #97. Source: openspec/changes/target-ontology-adapter/tasks.md §5.

Tasks

• 5.1 GraphWriter Protocol
• 5.2 Neo4jGraphWriter concrete impl
• 5.3 InMemoryGraphWriter test double
• 5.4 TargetModelMaterializer.write
• 5.5 Entity writes with compact enrichment_<facet>_status props
• 5.6 Property writes
• 5.7 Term writes (only when present)
• 5.8 Constraint writes
• 5.9 TargetObligation writes
• 5.10 EnrichmentDecision node + :HAS_ENRICHMENT_DECISION
• 5.10a Edge-endpoint Constraint nodes
• 5.10b EnrichmentStatusDivergenceError integrity check
• 5.11 Idempotent MERGE keys
• 5.12 Cypher migration (label, rel, uniqueness, indexes)
• 5.13 Reversible down-migration
• 5.14 InMemory writer sequence tests
• 5.15 Idempotency tests
• 5.16 model_role=TARGET on every write
• 5.17 Integration: golden-manifest round-trip
• 5.18 Integration: idempotency in Neo4j
• 5.19 Integration: two snapshot-hash generations coexist
• 5.20 Integration: enrichment-status indexed query (EXPLAIN)
• 5.20a Integration: edge-endpoint constraint payload