Settlement: add property-based test that sum of credits equals pool plus developer totals

[greatest0fallt1me]

Description

CalloraSettlement tracks a GlobalPool.total_balance and per-developer balances independently. There is no invariant test proving that the sum of all receive_payment amounts equals pool total + sum of developer balances. Add a randomized/property-style test in contracts/settlement/src/test.rs to catch accounting drift.

Requirements and Context

• Drive many randomized receive_payment calls (mix of to_pool=true/false) and assert the conservation invariant.
• Include overflow-boundary credits near i128::MAX.
• Add the invariant to INVARIANTS.md.
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/settlement-conservation-invariant

2. Implement changes
   • contracts/settlement/src/test.rs — invariant/property test
   • INVARIANTS.md — document the conservation invariant
3. Test and commit
   • cargo test -p callora-settlement
   • Cover overflow boundary
   • Include test output and notes in the PR

Example commit message

test: assert settlement credit conservation invariant

Acceptance Criteria

• Conservation invariant tested over many randomized credits
• Overflow boundary covered
• Invariant documented in INVARIANTS.md
• Tests pass

Guidelines

• .rs under contracts/settlement/src/, cargo test, /// docs, minimum 95% line coverage, no unwrap() in prod paths
• Clear documentation and inline comments
• Timeframe: 96 hours

[ZEMAD0N26]

@ZEMAD0N26 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Please kindly assign me to this issue, Thank you!

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Congratulations, @ZEMAD0N26! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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