Early Payment Discount on Platform Fee

[Kingsman-99]

Description

To incentivise prompt payment, the contract should offer a reduced platform fee for contributions made within a configurable early-bird window after invoice creation. Contributions outside this window pay the standard fee.

Technical Context

Add early_bird_window_ledgers: u32 and early_bird_fee_bps: u32 to the Invoice struct in types.rs. In the contribute method in contracts/split/src/lib.rs, compare env.ledger().sequence() against created_ledger + early_bird_window_ledgers to select the applicable fee. Emit EarlyBirdPayment(invoice_id, payer, discount_amount) from events.rs when the discount applies.

Acceptance Criteria

• Contributions within the early-bird window use early_bird_fee_bps
• Contributions outside the window use the standard platform fee
• early_bird_fee_bps must be less than or equal to the standard fee at creation time
• EarlyBirdPayment event is emitted with the calculated discount amount
• Window of zero ledgers disables the early-bird discount
• All CI checks (cargo build --target wasm32-unknown-unknown, cargo test, cargo clippy) pass and the branch has no merge conflicts

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[samuelfrancis163-eng]

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Hello, I'd like to work on this issue because i am confident i can deliver smoothly, Please assign issue to me

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[marvs8]

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hi maintainer,please i'll love to work on this issue

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[DevRushd]

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I would love to fix this issue. Can you please assign it to me.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@marvs8's PR #536 was approved and merged by @Kingsman-99.

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👏 Great work, @marvs8! Keep contributing to Stellar-split.

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