Add unit and integration tests for VexCandidateEmitter and SmartDiff repositories

- Implemented comprehensive unit tests for VexCandidateEmitter to validate candidate emission logic based on various scenarios including absent and present APIs, confidence thresholds, and rate limiting.
- Added integration tests for SmartDiff PostgreSQL repositories, covering snapshot storage and retrieval, candidate storage, and material risk change handling.
- Ensured tests validate correct behavior for storing, retrieving, and querying snapshots and candidates, including edge cases and expected outcomes.

src/Concelier/__Libraries/StellaOps.Concelier.Merge/AGENTS.md

@@ -8,7 +8,7 @@ Deterministic merge and reconciliation engine; builds identity graph via aliases
 - Merge algorithm: stable ordering, pure functions, idempotence; compute beforeHash/afterHash over canonical form; write merge_event.
 - Conflict reporting: counters and logs for identity conflicts, reference merges, range overrides.
 ## Participants
-- Storage.Mongo (reads raw mapped advisories, writes merged docs plus merge_event).
+- Storage.Postgres (reads raw mapped advisories, writes merged docs plus merge_event).
 - Models (canonical types).
 - Exporters (consume merged canonical).
 - Core/WebService (jobs: merge:run, maybe per-kind).
@@ -29,7 +29,7 @@ Out: fetching/parsing, exporter packaging, signing.
 - Logs: decisions (why replaced), keys involved, hashes; avoid dumping large blobs; redact secrets (none expected).
 ## Tests
 - Author and review coverage in `../StellaOps.Concelier.Merge.Tests`.
-- Shared fixtures (e.g., `MongoIntegrationFixture`, `ConnectorTestHarness`) live in `../StellaOps.Concelier.Testing`.
+- Shared fixtures (e.g., `PostgresIntegrationFixture`, `ConnectorTestHarness`) live in `../StellaOps.Concelier.Testing`.
 - Keep fixtures deterministic; match new cases to real-world advisories or regression scenarios.
 
 ## Required Reading