[contracts] Add flash-loan contract with reentrancy guard example

[CelestinaBeing]

Context

contracts/flashloan-token/ exists. Complete it with a reentrancy guard implementation and demonstrate both vulnerable and protected patterns.

Work

• Add flash_loan_unsafe(amount, callback_contract) without reentrancy protection (triggers S-reentrancy)
• Add flash_loan_safe(amount, callback_contract) using the Reentrancy Guard
• Add a mock callback contract for tests
• Demonstrate the attack in tests: malicious callback re-enters and drains
• Show sanctifier analyze catching the vulnerability

Acceptance criteria

• Attack contract successfully drains flash_loan_unsafe in test
• flash_loan_safe is not drainable
• sanctifier analyze finding count: 0 on safe, ≥1 on unsafe