Feat: amm module

[Agbasimere]

Closes: #28

Description

This PR implements a simple, robust Automated Market Maker (AMM) based on the Constant Product ($X \times Y = K$) formula. It allows users to provide liquidity to token pairs and swap between them with a 0.3% fee.

Features

• Liquidity Management: Proportional deposit and withdraw mechanisms for LP participants.
• Constant Product Swaps: Standard swapping logic with enforced slippage protection (min_amount_out).
• Standardized Fees: Integrated 0.3% trading fee to incentivize liquidity providers.
• Canonical Sorting: Ensures token order is consistent regardless of how the pool is initialized.
• Soroban SDK v22: Fully updated and tested against the latest project standards.

Integration

The AMM module is implemented as a standalone workspace member in src/amm:

• Add anchorpoint-amm = { path = "../amm" } to your contract's Cargo.toml.
• Deploy the AMM contract and call initialize(token_a, token_b) to create a new pool.

Commands

• Build/Check: cargo check -p anchorpoint-amm
• Unit Tests: cargo test -p anchorpoint-amm

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[Ceejaytech25]

@Agbasimere , lgtm!