Add invoice creator reputation staking

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Creators with no skin in the game can abandon invoices without consequence. This issue adds optional stake_amount: i128 to create_invoice() — the creator deposits USDC as a reputation bond. If the invoice is cancelled by the creator, the stake is distributed equally among payers as compensation.

Technical Context

Involves types.rs (add stake_amount: i128) and lib.rs. In create_invoice(), if stake_amount > 0, transfer from creator to contract. In cancel_invoice() (Issue 4), if stake exists, distribute stake_amount / unique_payer_count to each unique payer. On successful release, return stake to creator.

Acceptance Criteria

• create_invoice() transfers stake from creator when stake_amount > 0
• On creator cancellation, stake distributed equally among unique payers
• On successful release, stake returned to creator
• stake_amount = 0 produces identical behaviour to current implementation
• Test verifies payer receives stake share on creator cancellation
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Georgebingi]

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[sonoflawal]

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[dotmantissa]

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I can fix this.
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Kindly assign it to me.

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[zeemscript]

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[drips-wave]

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

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[drips-wave]

This issue has been completed by @sonoflawal as part of the Stellar Wave Program's 5th Wave 🥳

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