feat: add multi-sig arbitrator support — require M-of-N signatures for dispute resolution

[MarcusDavidG]

Summary

Allow invoice creators to specify a list of arbitrator addresses instead of a single one, with a configurable threshold (e.g. 2-of-3 must agree) to resolve disputes.

Motivation

Single-arbitrator escrow is a single point of failure. Multi-sig arbitration makes dispute resolution more trustless and suitable for DAO governance.

Proposed Change

• Add arbitrators: Vec<Address> and arbitrator_threshold: u32 to InvoiceOptions
• Track signatures in a new ArbitrationState storage entry
• Only execute resolve when threshold is met
• Add sign_arbitration(invoice_id) function

Acceptance Criteria

• Multiple arbitrators can sign
• Resolution only executes at threshold
• test: 1-of-1 behaves like current single arbitrator
• test: 2-of-3 requires 2 signatures before resolving
• test: duplicate signatures rejected