Add explicit overflow guard for r * bps remainder product in compute_share

[thlpkee20-wq]

Description

compute_share computes let remainder_product = r * bps; without a checked multiply, relying on the comment that r is bounded to (-10_000, 10_000) and bps <= 10_000. While that holds for the validated path, the public method returns early for bps > 10000 but the unchecked multiply is fragile to refactors. Replace it with checked_mul and add an explicit invariant test to lock the bound.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/lib.rs (compute_share), src/test_compute_share_invariants.rs
• Result must remain in [min(0,amount), max(0,amount)] for all inputs

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/compute-share-remainder-checked-mul
• Implement changes
  • Use r.checked_mul(bps) with a saturating fallback consistent with the base path
  • Add property assertions for the 0 <= result <= amount invariant
  • Document the decomposition bound formally in the doc comment
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test
• Cover edge cases
  • i128::MAX amount, bps 1/9999/10000, both rounding modes, negative amounts
• Include test output and security notes

Example commit message

feat: harden compute_share remainder math with checked multiply

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[millicentogalanya]

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

please assign this issue to me, i've worked on something similar and i'll deliver quickly. ETA~5hrs

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

[drips-wave]

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

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