Add Swap Counterparty Reputation Tracking

[fejilaup-cloud]

Description

There is no way to track buyer/seller reputation. Untrustworthy counterparties can repeatedly default without consequences.

Tasks

• Add DataKey::UserReputation(Address) to store reputation score (0-100)
• Implement get_reputation(env, address) -> u32
• Update reputation on swap completion: +5 for successful, -10 for cancelled/disputed
• Implement set_reputation_multiplier(env, min_reputation) — seller can require minimum buyer reputation
• Add tests for reputation tracking and enforcement

Priority: Medium | Estimated Time: 2 hours