docs(book): Add Section 19.13 Worked Example - Cross-DEX Flash Loan Arbitrage

- Complete real-world example: SOL/USDC arbitrage on Orca vs Raydium
- Opportunity detection: 1.73% spread ($98.50 vs $100.20)
- Stage-by-stage execution: validation → allocation → profitability → execution
- Multi-oracle validation: 3 sources agree (Pyth, Chainlink, DEX TWAP)
- Multi-pool optimization: Balancer 0% fee (saved $8.55 vs Aave)
- Reentrancy protection: Mutex acquired/released successfully
- Final result: 0.88 SOL profit (17.6% ROI in 0.68 seconds)
- Complete accounting: 5 SOL → 5.88 SOL with 20x leverage
- Demonstrates all safety mechanisms working in production
- Chapter 19: ~11,000 → ~13,400 words (+22%)

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Co-Authored-By: Claude <[email protected]>

Author: 0xrinegade