[CT-25] Complete atomic swap functionality in multisig_transfer in opsce crate

[yusuftomilola]

Problem

contracts/multisig_transfer/ handles asset transfers but has no atomic swap capability. Two parties cannot atomically exchange different assets in a single transaction, meaning each leg must be executed separately and introduces counterparty risk.

Proposed Solution

Create contracts/opsce/src/atomic_swap.rs. Implement a two-phase commit swap: both parties lock their assets into the contract, then a single execute_swap() call transfers both legs atomically.

Acceptance Criteria

• propose_swap(env, asset_a_id, asset_b_id, amount_a, amount_b, counterparty) locks Asset A from the proposer
• accept_swap(env, swap_id) locks Asset B from the counterparty
• execute_swap(env, swap_id) transfers Asset A to counterparty and Asset B to proposer atomically — either both succeed or neither does
• cancel_swap(env, swap_id) returns locked assets to their owners — can be called by either party before execute_swap
• SwapStatus enum: Proposed, Accepted, Executed, Cancelled
• Emits swap_proposed, swap_accepted, swap_executed, swap_cancelled events
• Unit tests cover: full happy path, cancellation by proposer, cancellation by counterparty, re-execution attempt

[ummarig]

@ummarig has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I please be assigned this task

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ummarig to this issue.

[drips-wave]

Congratulations, @ummarig! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @ummarig: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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