Add time-based interest accrual and debt indexes

[zhero-o]

Introduces index-based accounting (Aave/Compound-style) so borrow and supply balances grow over time instead of remaining static. A new InterestState (supply_index/borrow_index/last_accrual_timestamp) is tracked per asset, with per-position index snapshots realized into Position.deposited/borrowed on every touch. The rate model is a simple, deterministic linear function of utilization (derived from the existing TotalDeposited/TotalBorrowed totals), with supply_rate = borrow_rate * utilization so interest accrued to borrowers always equals interest credited to suppliers (no protocol fee skim). All accrual math lives in interest.rs as pure, storage-free functions with dedicated unit tests; deposit/borrow/repay/withdraw/record_protocol_fee accrue before their own effects so caps and balances stay accurate, and get_position/ get_asset_reserve reflect live simulated values without persisting. Bumps CONTRACT_VERSION/STORAGE_SCHEMA_VERSION to 2 (VLENDV2) per the project's own versioning convention, since storage layout changed.

Along the way, fixed two pre-existing bugs blocking this work:

• tests/integration.rs was configured with harness = false and an empty main(), silently disabling all 10 integration tests (they never compiled or ran, and used an incorrect contract-registration pattern that wouldn't have compiled anyway).
• configure_asset() wrote deposit/borrow caps and totals as untyped integer literals, which Rust defaults to i32, while every reader expects i128 - any configured asset's caps/totals panicked with a storage type mismatch as soon as they were read back.

Closes #174