Profit fee formula golden vectors from docs

[Baskarayelu]

Description

Add table-driven tests matching docs/contracts/profit-fee-formula.md examples.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Align with existing Soroban contract patterns in quicklendx-contracts

Suggested execution

• Fork the repo and create a branch
• git checkout -b feature/profit-fee-golden-vectors
• Implement changes
• Write contract: src/profits.rs
• Write comprehensive tests: src/test_profit_fee_formula.rs, src/test_profit_fee.rs
• Add documentation: docs/contracts/profit-fee-formula.md
• Include NatSpec-style comments (Rust doc comments on public items)
• Validate security assumptions

Test and commit

• Run tests
• Cover edge cases
• Include test output and security notes

Example commit message

test: add profit fee golden vectors from docs

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours