[contracts] Add a comprehensive DEX (AMM) contract with full SEP-41 compliance

[CelestinaBeing]

Context

contracts/amm-pool/ exists but may not be fully SEP-41 compliant or complete. A reference DEX contract would showcase Sanctifier's analysis of a complex real-world contract.

Work

• Complete contracts/amm-pool/ with: swap, add_liquidity, remove_liquidity, get_price functions
• Ensure full SEP-41 token interface compliance for LP tokens
• Add overflow protection on all arithmetic (Sanctifier must show 0 S003 findings)
• Write extensive integration tests covering edge cases (zero liquidity, price impact, etc.)
• Add a README.md explaining the AMM math

Acceptance criteria

• sanctifier analyze contracts/amm-pool shows 0 critical or high findings
• All SEP-41 functions present and tested
• AMM math verified correct by Z3 invariant (k = x * y)