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.

docs/benchmarks/scanner-feature-comparison-trivy.md

@@ -29,7 +29,7 @@ _Reference snapshot: Trivy commit `012f3d75359e019df1eb2602460146d43cb59715`, cl
 
 | Dimension | StellaOps Scanner | Trivy |
 | --- | --- | --- |
-| Architecture & deployment | WebService + Worker services with queue abstraction (Redis Streams/NATS), RustFS/S3 artifact store, Mongo catalog, Authority-issued DPoP tokens, Surface.* libraries for env/fs/secrets, restart-only analyzer plugins.[1](#sources)[3](#sources)[4](#sources)[5](#sources) | Single Go binary CLI with optional server that centralises vulnerability DB updates; client/server mode streams scan queries while misconfig/secret scanning stays client-side; relies on local cache directories.[8](#sources)[15](#sources) |
+| Architecture & deployment | WebService + Worker services with queue abstraction (Redis Streams/NATS), RustFS/S3 artifact store, PostgreSQL catalog, Authority-issued DPoP tokens, Surface.* libraries for env/fs/secrets, restart-only analyzer plugins.[1](#sources)[3](#sources)[4](#sources)[5](#sources) | Single Go binary CLI with optional server that centralises vulnerability DB updates; client/server mode streams scan queries while misconfig/secret scanning stays client-side; relies on local cache directories.[8](#sources)[15](#sources) |
 | Scan targets & coverage | Container images & filesystem snapshots; analyser families:<br>• OS: APK, DPKG, RPM with layer fragments.<br>• Languages: Java, Node, Python, Go, .NET, Rust (installed metadata only).<br>• Native: ELF today (PE/Mach-O M2 roadmap).<br>• EntryTrace usage graph for runtime focus.<br>Outputs paired inventory/usage SBOMs plus BOM-index sidecar; no direct repo/VM/K8s scanning.[1](#sources) | Container images, rootfs, local filesystems, git repositories, VM images, Kubernetes clusters, and standalone SBOMs. Language portfolio spans Ruby, Python, PHP, Node.js, .NET, Java, Go, Rust, C/C++, Elixir, Dart, Swift, Julia across pre/post-build contexts. OS coverage includes Alpine, RHEL/Alma/Rocky, Debian/Ubuntu, SUSE, Amazon, Bottlerocket, etc. Secret and misconfiguration scanners run alongside vulnerability analysis.[8](#sources)[9](#sources)[10](#sources)[18](#sources)[19](#sources) |
 | Evidence & outputs | CycloneDX (JSON + protobuf) and SPDX 3.0.1 exports, three-way diffs, DSSE-ready report metadata, BOM-index sidecar, deterministic manifests, explain traces for policy consumers.[1](#sources)[2](#sources) | Human-readable, JSON, CycloneDX, SPDX outputs; can both generate SBOMs and rescan existing SBOM artefacts; no built-in DSSE or attestation pipeline documented—signing left to external workflows.[8](#sources)[10](#sources) |
 | Attestation & supply chain | DSSE signing via Signer → Attestor → Rekor v2, OpenVEX-first modelling, lattice logic for exploitability, provenance-bound digests, optional Rekor transparency, policy overlays.[1](#sources) | Experimental VEX repository consumption (`--vex repo`) pulling statements from VEX Hub or custom feeds; relies on external OCI registries for DB artefacts, but does not ship an attestation/signing workflow.[11](#sources)[14](#sources) |