[Contract] Issue #8: Safe Math for Accrued Yield Compounding

[ONEONUORA]

Implement a safe compounding function for interest calculations. Use checked math i128.checked_mul and i128.checked_div to prevent arithmetic overflows when computing yield returns based on yield_rate_bps.

[martinvibes]

can i work on this issue

[grantfox-oss]

🦊 GrantFox — @martinvibes has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #919)
2. Your PR will be reviewed by the Fracverse maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@martinvibes's PR #966 was approved and merged by @ONEONUORA.

🏆 @martinvibes: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (274 total points). Track your full progress on GrantFox.

👏 Great work, @martinvibes! Keep contributing to Fracverse.