Implement deposit reward auto-compounding for stability pool

[Kevin737866]

Description

Automatically compound stability pool rewards by reinvesting earned rewards into the pool, maximizing depositor returns.

Acceptance Criteria

• Rewards auto-compounded at configurable intervals
• Compound events with before/after balances
• User can opt-out of auto-compounding
• Gas-efficient batch compounding
• Reward index updated correctly after compound

Files to Modify

• src/contracts/stability_pool.rs -- add auto-compound
• tests/ -- compound tests