feat: add internal verification functions to IdentityManagerV2

[tyranis0x01]

Summary

This PR introduces new internal functions to the IdentityManagerV2 contract that improve modularity and code reuse when handling verification logic.

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Changes

src/IdentityManagerV2.sol

• Added Internal Functions Natspec Section Header
• Implemented _storeVerification:
  • Stores full user verification data (user type, level, timestamps, metadata, nullifierHash)
  • Updates tracking arrays and platform statistics
• Implemented _directVerify:
  • Allows admins to directly verify users internally
  • Reuses _storeVerification to enforce consistency
  • Emits UserVerified event

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Benefits

• Improves code modularity by centralizing verification storage logic
• Prevents duplication across future verification flows
• Enables internal admin verification without exposing additional external functions

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Next Steps

• Integrate _storeVerification into external verification flows
• Add unit tests for _directVerify and _storeVerification behavior
• Extend metadata handling for richer user verification records