4. Add time-based interest accrual and debt indexes

[Cedarich]

Area: Soroban
Complexity: Hard

Summary:
Introduce index-based accounting so debt and supply positions can evolve over time instead of remaining static balances.

Acceptance Criteria:

• Borrow and supply accounting uses time-aware indexes.
• State transitions remain deterministic and testable.
• Core accrual behavior is covered by host-side tests.

[Ayinkx]

Hi i would like to work on this issue please assign to me
I can do it perfectly

[zhero-o]

Hello, I would like to work on this issue. can i get assigned?

[grantfox-oss]

🦊 GrantFox — @zhero-o 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 #174)
2. Your PR will be reviewed by the Zyntarivoid maintainers

Good luck! Track your progress on GrantFox.