feat(verisim): scaffold VeriSimDB octad bridge for proof metadata storage

- ProofOctad struct with 8 modalities (Prover, Trust, Axioms, Time, Certificates, Tactics, Dependencies, Confidence)
- VeriSimDBClient interface (scaffolding, awaits verisimdb-phase-1 API)
- CorpusHealthSnapshot for corpus metrics via VeriSimDB
- 5 unit tests validating octad creation, serialization, client initialization
- Feature-gated behind [features] verisim flag (not enabled by default)
- Ready for API integration once VeriSimDB public interface stabilizes

Integration points marked with TODO(verisimdb-phase-1) for future wiring:
- prove_handler → store_octad() after successful proof
- dispatch.rs confidence_scoring → update_confidence()
- corpus_monitor → corpus_health_snapshot()

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: hyperpolymath