Build invoice group all-or-nothing release

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Some workflows require multiple invoices to all be fully funded before any of them release — for example a multi-party deal where all parties must commit before funds move. This issue adds create_invoice_group() linking invoices so none can release until all are fully funded.

Technical Context

Involves lib.rs. Add persistent key ("group", group_id) storing Vec<u64> of invoice IDs. Add create_invoice_group(env, invoice_ids: Vec<u64>) -> u64. In release(), if invoice belongs to a group, check all group members are fully funded before releasing any.

Acceptance Criteria

• create_invoice_group() links multiple invoice IDs
• release() on a group member panics if any other member is not fully funded
• All group members release successfully once all are funded
• Non-grouped invoices behave identically to current implementation
• Test creates group of 2 invoices, funds both, verifies both release
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[marvs8]

@marvs8 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hi maintainer

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Congratulations, @marvs8! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @marvs8: 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 completed by @marvs8 as part of the Stellar Wave Program's 5th Wave 🥳

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