Add flash loan integration with AMM arbitrage support

[Smartdevs17]

Description

Implement flash loan functionality that integrates with AMM pools for arbitrage, with proper fee collection and reentrancy protection.

Acceptance Criteria

• Flash loan contract with callback pattern
• Fee calculation (0.09% base + dynamic adjustment)
• AMM integration for atomic arbitrage
• Maximum flash loan amount based on pool liquidity
• Flash loan analytics dashboard
• Reentrancy verification with formal analysis

Technical Scope

• stellar-lend/contracts/lending/src/flash_loan.rs
• stellar-lend/contracts/amm/src/
• stellar-lend/contracts/hello-world/src/flash_loan.rs
• Edge: callback reentrancy, gas limit on complex callbacks, price impact