Contracts: Add safe math guards and overflow assertions for fee calcu…

[midenotch]

Close: #556

Pull Request Template

📋 Description

I updated lib.rs (line 194) so accrue_yield now returns Result<(), VaultError>, rejects non-positive amounts, reports checked math failures as VaultError::MathOverflow, and calculates fee/net yield before transferring tokens. I also added focused regression coverage in test.rs (line 412) for zero amount, fee math overflow before transfer, and 100% fee routing to treasury.
Verification:
cargo test -p vault accrue_yield -- --nocapture passed: 4 tests.
cargo check --workspace --all-features passed.
Full cargo test -p vault and cargo test -p vault --lib both timed out after 5 minutes, likely due the broader/property-heavy suite. I cleaned up the snapshot/formatting side effects from those runs.

🔗 Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 📚 Documentation update
• 🔒 Security improvement

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🔒 SECURITY REVIEW (⭐ MANDATORY FOR SMART CONTRACT CHANGES)

For all smart contract code changes, complete the following checklist.

See docs/SECURITY_CHECKLIST.md for detailed guidance.

Required: Security Checklist Sign-Off

• I have reviewed this PR against the Internal Security Checklist (docs/SECURITY_CHECKLIST.md)

  • Reentrancy: Verified Checks-Effects-Interactions (CEI) pattern
  • Access Control: Confirmed all sensitive functions are protected (onlyOwner, onlyRole(), etc.)
  • Input Validation: Validated all parameters have appropriate bounds checks
  • Unchecked Returns: All external calls have return value checks (require(success, ...))
  • Gas Limits: No unbounded loops or potential DOS vectors

  If any checkbox cannot be verified, explain below:

  N/A — this PR contains only documentation changes. No smart contract code was modified.
  

Slither Static Analysis Results

• Ran Slither locally: slither . --config-file slither.config.json

  • Result: ✅ No High/Medium findings OR 🟡 Documented false positives (see below)

• GitHub Actions Slither workflow passed:

  • 🟢 All High/Medium findings fixed OR
  • 🟡 All false positives documented with FP references

  If this PR has security findings, document them below:

  N/A — documentation-only PR. No contract or runtime code changed.
  

Handling Security Findings

Option A: Fixed in This PR ✅

• Vulnerability identified and resolved
• Test case added to verify fix
• Explain fix below:

  N/A
  

Option B: False Positive 🟡

• Identified as false positive (tool limitation or misleading check)
• Added entry to contracts/.false-positives.md with:
  • Detector rule name
  • Technical reasoning (3+ sentences why it's safe)
  • Evidence (code snippet, test case, or reference)
• Reference number (e.g., FP-001):

  N/A
  

• Inline suppression added to code:

  // slither-disable-next-line <detector-name>
  // Reason: [one-line reason]

Option C: Accepted Risk ⚠️

• Acknowledged as low-priority style issue (naming conventions, etc.)
• Added to Slither exclusions
• Explain below:

  N/A
  

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📝 Testing

Functional Testing

• Unit tests added/updated for changes
• Integration tests passing
• Manual testing completed and documented below:

  - Verified all variable names, defaults, and required flags against source files:
    backend/src/index.ts, rateLimiter.ts, auth.ts, tracing.ts
  - Cross-checked every .env.example, .env.local.example, .env.production.example
    in both backend/ and frontend/
  - Confirmed links in README.md and ENV_QUICK_REFERENCE.md resolve correctly
  - No runtime code changed; no functional regression possible
  

Security Testing

• For state-changing functions:

  • Reentrancy test (if applicable): Verify re-entry is blocked
  • Access control test: Verify unauthorized access is rejected
  • Boundary test: Verify edge cases are handled

• For external integrations:

  • Return value verification test
  • Failure scenario test

Test Coverage

• All new code paths have test coverage
• Security-critical paths have comprehensive test cases
• Coverage report: N/A — documentation only, no executable code added

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🚀 Deployment Notes

No deployment steps required. This PR adds a Markdown file and updates two existing Markdown files only.

Mainnet Readiness

• This code is ready for production deployment
• All critical tests pass
• Security review approved
• No temporary debug code
• No TODO comments

Breaking Changes

If this PR introduces breaking changes:

• Migration guide provided
• Deprecation period defined: [timeframe]
• Legacy code deprecated with warnings

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📊 Automated Scan Results

Slither Analysis

• ✓ Status: N/A — no contract code changed
• 🔴 High/Medium findings: 0
• 🟡 Low/Informational findings: 0
• 🟢 No issues detected: documentation-only PR

Related Documentation

• Security Checklist — Use for code review
• False Positive Process — For non-vulnerabilities
• Slither Configuration — Current scanner settings

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✅ Reviewer Checklist

For code reviewers (use this to guide your security-focused review):

• PR author completed security checklist ✓
• All findings documented and categorized (fixed/false positive/excluded)
• Inline security comments are clear and justified
• Tests cover security-critical code paths
• No external calls bypass return value checks
• Access control is properly enforced
• State updates follow CEI pattern
• Input validation is comprehensive
• Follow-up actions (if any) tracked in issues

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📞 Questions or Issues?

• 🤔 Confused about security checklist? → See docs/SECURITY_CHECKLIST.md
• 🔍 Marking finding as false positive? → Follow docs/FALSE_POSITIVE_HANDLING.md
• 🆘 Need security review help? → Tag @security-team in comments

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📋 Pre-Submit Checklist

Before marking PR as ready for review:

• Description is clear and concise
• All security checklist items checked (✅ or explanation provided)
• All tests passing locally: npm test
• Linter passing: npm run lint
• Slither passing locally OR findings documented: slither . --config-file slither.config.json
• Code follows project style guide
• No merge conflicts
• Commits are clean and well-documented
• Branch is up-to-date with main/develop

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✅ Ready for Review? Ensure all items above are checked before requesting review.

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@midenotch Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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[midenotch]

Done, Close: #667