fix: accrue interest with bps denominator

[silentgeckoaudit3801]

Closes #1307

Summary:

• Use the standard 10,000 basis-point denominator for interest accrual instead of 100,000.
• Add a regression test for a 10,000 principal loan at 1,200 bps over one full default term, expecting 1,200 accrued interest.

Validation:

• Static GitHub API/source inspection only; local Soroban/Rust toolchain commands were not run to avoid installing or executing project dependencies in this environment.