Introduce HubService struct with types for hub coordination: PendingOp,
HubClaim, LeaderboardEntry, GlobalStats, ConflictError, NotFoundError.
Constructor generates a crypto/rand client ID when none exists. Includes
no-op loadPendingOps/savePendingOps stubs for future persistence.
Co-Authored-By: Virgil <virgil@lethean.io>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
