Add flash-loan price manipulation test vector

[MyTH-zyxeon]

Summary

• add a copy-pasteable FlashLoanPriceManip.t.sol end-to-end exploit template
• model a UniV2-style spot-price pair, vulnerable lending vault, flash-loan pool, and attacker flow
• document the starter example in README.md and CONTRIBUTING.md

Closes #20

Validation

• forge fmt --check test/FlashLoanPriceManip.t.sol
• forge test --match-path test/FlashLoanPriceManip.t.sol -vvv (1 passed; logs show spot price 1.0 -> 35.91 and 500 quote token profit)
• git diff --check

[abhicris]

🤖 Audit verdict: safe

Legitimate security education PR adding a well-documented flash-loan price manipulation test vector to an audit-checklist repository; mock contracts are clearly intentional demonstrations without malicious payload, credential leakage, or dangerous bugs.

Audited by the kcolbchain PR pipeline. See pipeline docs.