docs: update TestBorrower address after security fix redeployment

[devin-ai-integration]

Summary

Updates the README with the new TestBorrower contract address after redeploying with the security fix from PR #22 (hardening against arbitrary lender injection attacks).

• Old TestBorrower: 0x53cddbcdee2dc2b756a25307f4810c609b28c3e7 (vulnerable)
• New TestBorrower: 0x7e13a21ce933a7122a8d1bdf0aeced4ba48ecad6 (fixed)
• Deploy tx: 0x6fbf0987...

Also added a note that the gas breakdown numbers (85,292) are from the legacy TestBorrower.

Review & Testing Checklist for Human

• Verify the new TestBorrower address - Confirm 0x7e13a21ce933a7122a8d1bdf0aeced4ba48ecad6 is the correct deployment
• Verify contract on Etherscan - Automated verification failed due to compiler version mismatch. Run locally:

  forge verify-contract 0x7e13a21ce933a7122a8d1bdf0aeced4ba48ecad6 src/TestBorrower.sol:TestBorrower --etherscan-api-key YOUR_KEY --watch

• Consider updating gas numbers - The new TestBorrower has additional security checks (expectedLender, expectedAmount validation) which will increase gas usage. A new flash loan tx would give accurate numbers.

Notes

The new TestBorrower includes security hardening from PR #22:

• Validates msg.sender == expectedLender
• Validates initiator == address(this)
• Validates token == USDC
• Validates amount == expectedAmount

Link to Devin run: https://app.devin.ai/sessions/55585942721f4bcaa48cdfa0c67d9110
Requested by: Player 53627 (github.stagnate430@passmail.com) / @igor53627

[devin-ai-integration]

Original prompt from Player 53627

https://github.com/igor53627/liq/blob/main/research/BALANCER_COMPARISON.md should we update this file? do you need Tenderly keys for test? 

You only need to look in the following repo: igor53627/liq

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