feat: Add verification stats and nullifier check view functions

[malgus01]

PR Description

Summary

This PR enhances the IdentityManagerV2 contract with new external view functions that improve transparency and usability for external integrations.

Changes

• IdentityManagerV2
  • Added getVerificationStats():
    • Returns current platform verification statistics (VerificationStats struct).
    • Enables external consumers (frontends, analytics tools) to easily query platform metrics.
  • Added isNullifierUsed(uint256 nullifierHash):
    • Returns whether a given nullifier has been used.
    • Enhances ZK verification workflows by allowing external checks before submission.
  • Added full NatSpec documentation for both functions.
• Ran forge fmt for consistent formatting.

Impact

• Improves developer experience by exposing critical read-only information through clean external view functions.
• Facilitates integrations with off-chain services and user interfaces.
• Reduces reliance on internal state inspection for common verification use cases.

Next Steps

• Consider extending VerificationStats with more granular metrics (e.g., active users, revoked identities).
• Add tests to validate correct behavior of new view functions under edge cases.
• Integrate isNullifierUsed checks in frontend flows to prevent duplicate submissions.