Add recipient vesting cliff on release

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Recipients currently receive their full share the moment an invoice is released. This issue adds an optional vesting_cliff: Option<u64> (ledger timestamp) per invoice — funds are held in the contract until the cliff passes, after which any address can call claim(invoice_id, recipient) to transfer the recipient's share.

Technical Context

Involves types.rs (add vesting_cliff: Option<u64>, claimed: Vec<bool>), lib.rs. In _release(), if cliff is set, mark status Released but do not transfer. Add claim(env, invoice_id, recipient) that checks cliff passed and claimed[i] is false, then transfers.

Acceptance Criteria

• _release() holds funds when cliff is set
• claim() before cliff panics with "cliff not reached"
• claim() after cliff transfers correct amount to recipient
• Each recipient can only claim once
• No cliff behaves identically to current implementation
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Swayymalcolm99]

@Swayymalcolm99 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer! I have experience implementing vesting cliff logic in token release contracts, including cliff timestamp enforcement, pre-cliff release blocking, and correct cliff-to-schedule transition handling with full test coverage.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Swayymalcolm99 to this issue.

[Saboleee]

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

Hi Emmy please assign me this issue

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

[drips-wave]

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

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