feat: add verification stats and nullifier tracking view functions

[tyranis0x01]

Summary

This PR extends IdentityManagerV2 with additional verification analytics and nullifier usage checks to improve transparency and integration support.

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Changes

src/IdentityManagerV2.sol

• New view functions

  • getVerificationStats() → returns current platform verification statistics
  • isNullifierUsed(uint256 nullifierHash) → checks whether a given nullifier has already been used

• Documentation

  • Added NatSpec comments for new functions

• Maintenance

  • Ran forge fmt for formatting consistency

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Benefits

• Provides integrators with platform-wide verification insights
• Adds anti-replay protection visibility through nullifier checks
• Improves external dApps' ability to query system state without extra indexing

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Next Steps

• Expand VerificationStats struct with more detailed metrics if required
• Add unit tests for getVerificationStats and isNullifierUsed
• Integrate nullifier checks into end-to-end identity flows