Soroban: On-chain contribution router with slippage enforcement

[dotunv]

Problem

Path payment slippage is currently enforced client-side in the backend (SLIPPAGE_BPS = 500 hardcoded in contributions.js:14). The backend fetches a quote, calculates sendMax, and submits — but there is no on-chain guarantee the slippage rule is respected. A malicious or buggy backend could submit a transaction with no slippage protection.

A Soroban contract can enforce contribution routing rules on-chain, making the contribution flow trustless.

What needs to happen

Soroban Contract (contracts/soroban/router/)

Write a Rust Soroban contract that:

• Accepts: send_asset, send_max, dest_asset, dest_amount, path, campaign_wallet, platform_wallet, fee_bps
• Validates: send_max does not exceed dest_amount * (1 + max_slippage_bps / 10000)
• Executes a path_payment_strict_receive on-chain
• Splits the received amount: (1 - fee_bps / 10000) to campaign wallet, fee_bps / 10000 to platform wallet — atomically in one call
• Emits a ContributionRouted event with: sender, campaign, dest_amount, source_amount, fee_amount, path

Backend changes

• Replace prepareSignedContributionPathPayment() in stellarService.js with a contract invocation
• Fee split (Issue Feature: Platform fee collection on contributions #20) moves into the contract — no separate operation needed
• Backend indexes ContributionRouted events instead of raw path_payment_strict_receive operations

Why atomic fee split matters

Currently the fee would need to be a separate transaction or operation. The router contract does it in one atomic call — the campaign either receives the full net amount or nothing. No partial states.

Acceptance criteria

• Slippage ceiling enforced at contract level, not backend
• Fee split to platform wallet atomic with the contribution
• ContributionRouted event emitted and indexable
• Existing contribution UI works unchanged
• Path payment quote still fetched from Horizon (off-chain is fine for quotes)
• Contract rejects transactions that exceed slippage threshold

Dependencies

• Depends on Issue Feature: Platform fee collection on contributions #20 (platform fee) — fee logic moves into this contract
• Works alongside Issue Soroban: Trustless on-chain escrow contract to replace backend-controlled multisig #58 (escrow contract)

[Rocket1960]

@Rocket1960 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Rocket1960 to this issue.

[drips-wave]

Congratulations, @cybermax4200! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @cybermax4200: 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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[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @cybermax4200 as part of the Stellar Wave Program's 4th Wave 🥳

Moderator's reason: [Auto-assessed] The contributor successfully implemented the Soroban router contract with on-chain slippage enforcement and atomic fee splitting, as well as the necessary backend integration with a backward-compatible fallback. The PR includes comprehensive Rust unit tests with snapshots and fixes existing backend tests, representing a high-quality contribution that fully addresses the issue requirements.

😎 @cybermax4200: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.