Implement configurable stream cancellation fee payable to the protocol treasury

[Chuks-coderr]

Overview

Early cancellation of streams by senders provides no penalty, enabling fee-free capital extraction strategies where senders create and cancel streams speculatively.

Problem

Without a cancellation fee, senders can abuse the protocol's escrow function with no cost, effectively using it as a free temporary holding mechanism for tokens.

Proposed Solution

• Add an admin-configurable cancellation_fee_basis_points: u32 parameter
• On cancel_stream, deduct a percentage of the sender's unstreamed refund and transfer it to the protocol treasury
• No fee applied when the stream has already passed its end_time (natural expiry is free)
• Emit the fee amount in the CancelledStream event

Acceptance Criteria

• Admin can set cancellation_fee_basis_points
• cancel_stream deducts the fee from the sender's refund and credits the treasury
• No fee deducted when current_time >= end_time
• CancelledStream event includes cancellation_fee field
• Unit tests cover: fee deducted on early cancel, no fee on natural expiry, fee correctly calculated

[Evaristus023]

@Evaristus023 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I’ve thoroughly reviewed the requirements and will implement a clean, maintainable solution that fully meets the acceptance criteria, along with a well-structured pull request.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Evaristus023 to this issue.

[drips-wave]

Congratulations, @Abigail6168! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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