feat(dispute): add appeal window — allow losing party to contest resolution

[Leothosine]

Summary

Adds an appeal window after resolve_dispute. The claimant can call appeal_dispute within APPEAL_WINDOW (1000) ledgers to escalate to a new resolver.

Changes

• Add appeal_deadline_ledger: u32 to DisputeRecord
• Add DisputeStatus::Appealed variant
• resolve_dispute sets appeal_deadline_ledger = current_ledger + APPEAL_WINDOW
• Add appeal_dispute(e, appellant, dispute_id, new_resolver) — claimant only, within window
• Expose in contract.rs

closes #199

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