feat: add internal verification handling functions to IdentityManagerV2

[malgus01]

PR Description

Summary

This PR introduces internal helper functions in IdentityManagerV2 to streamline verification storage and direct admin verification flows.

Changes

• Internal Functions

  • Added _storeVerification:
    • Stores verification details for a user, including type, level, expiration, nullifier, and metadata.
    • Updates verification tracking arrays (verifiedUsers, usersByType).
    • Updates platform verification statistics.
  • Added _directVerify:
    • Provides an internal admin pathway for directly verifying users without requiring external proof.
    • Emits a UserVerified event upon success.

• Documentation

  • Added Natspec header section for internal functions.
  • Added detailed Natspec comments for each new function.

• Refactor

  • Centralized verification logic into _storeVerification to ensure consistency across verification flows.

Impact

• Simplifies verification logic and reduces code duplication.
• Provides an admin-only mechanism for direct verification in exceptional cases.
• Improves maintainability with clear separation between external and internal verification processes.

Next Steps

• Extend _directVerify for batch verification.
• Add revocation logic for direct/admin verifications.
• Expose new external functions leveraging these internals where appropriate.