Reconcile contract token balance with `funded_amount` to bound `sweep_terminal_dust` blast radius

[mikewheeleer]

Description

sweep_terminal_dust can move up to MAX_DUST_SWEEP_AMOUNT of the funding token to the treasury in terminal states, and the rustdoc warns integrations must keep token balances reconciled with funded_amount so the sweep cannot pull user principal. Add an on-chain invariant check that the sweep never reduces the contract balance below outstanding investor liabilities.

Requirements and context

• Scoped to the LiquiFact escrow Soroban contract.
• Before transferring, compute outstanding = funded_amount - (already-distributed) and assert balance - sweep_amt >= outstanding; otherwise trap.
• Track distributed principal explicitly (new storage key) so the invariant is computable on-chain.
• Invariant: a dust sweep can never reduce the balance below funds owed to unredeemed investors.
• Reference ADR-006 and docs/escrow-security-checklist.md.
• Must be secure, tested, and documented.

Suggested execution

• Fork the repo and create a branch:
  • git checkout -b security/dust-sweep-liability-floor
• Implement changes:
  • escrow/src/lib.rs
  • Tests: escrow/src/tests/external_calls.rs
  • Docs: docs/adr/ADR-006-dust-sweep-and-token-safety.md
  • Include rustdoc/NatSpec-style doc comments on public functions
  • Validate security assumptions (auth, overflow, storage TTL, double-spend)

Test and commit

• Run tests: cargo test
• Cover edge cases (zero amounts, overflow, unauthorized callers, double-spend, state-machine misuse)
• Include test output and security notes in the PR

Example commit message

feat(escrow): enforce liability floor on terminal dust sweep

Guidelines

• Minimum 95% test coverage on new/changed code
• Clear documentation
• Timeframe: 96 hours from assignment

[Vvictor-commits]

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

please assign this issue to me

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

[drips-wave]

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

🧑‍💻 @Vvictor-commits: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊