Implement snapshot-based voting weight to prevent flash voting …

[Mac-5]

…attacks

• Add dispute_snapshot_ledger field to Market struct to capture ledger sequence when dispute is filed
• Implement try_get_balance_at() to query governance token balance at snapshot ledger
• Add fallback mechanism that locks tokens for 3-day resolution period if token doesn't support snapshots
• Add InsufficientVotingWeight and GovernanceTokenNotSet error codes
• Add LockedTokens struct and unlock_tokens() function for fallback mechanism
• Add set_governance_token() admin function to configure governance token
• Comprehensive test suite covering:
  • Balance changes after snapshot (flash loan prevention)
  • Zero balance at snapshot rejection
  • Token locking fallback mechanism
  • Insufficient balance validation
  • Ledger sequence capture verification
    Closes Snapshot-Based Verifiable Voting Weight #3