M06 Error Handling

This cuts through Rust's error handling by asking the right questions first: is this failure expected or a bug? Instead of defaulting to unwrap or panic, it walks you through Option versus Result, thiserror versus anyhow, and when propagation with ? actually makes sense. The flowchart alone is worth it. It connects to domain modeling (m09) when you're unsure whether absence is normal, and distinguishes library code (use thiserror for typed errors) from application code (use anyhow for ergonomics). The "trace up" and "trace down" structure helps when you're stuck between too many unwraps or lost error context. Good for anyone past the basics who keeps second-guessing their error strategy.