Add interview questions

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

INTERVIEW_QUESTIONS.md

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+# Interview Questions
+
+## Before coding
+
+1. Walk me through the code. What does each file do, and how does a trade move through the system?
+2. Which assumptions would you check with me before you touch anything?
+3. CI is green but the ticket says the code is broken. What does that tell you, and where do you look first?
+4. How would you reproduce the ticket? Can you do it with a smaller input than `main.py` uses?
+
+## Logger
+
+5. The logs show "no errors". How would you prove whether an error really did not happen or it happened and was not logged?
+6. The log lines don't match their trades. Why could the context be correct when you log but wrong when the line is written?
+7. Why might someone buffer log records? What are the risks?
+8. How does Python's standard `logging` module avoid the same problems? Name the specific mechanisms.
+9. Would you replace this logger with `logging`? What would that cost, and what would you gain?
+10. How would you test a logger whose output contains a timestamp?
+
+## State and data model
+
+11. Who owns a `Trade` once it is passed to `Ledger.apply`? How would you enforce that in the code?
+12. Name two or more ways to stop callers from changing a `Trade`. What does each one cost?
+13. The EOD rebuild swallows `ValueError`. Is that the right policy? What would you change?
+14. Why did the existing rebuild test still pass?
+
+## Positions and P&L
+
+15. Work this out by hand: buy 100 @ 100, then sell 150 @ 105. What are `qty`, `avg_price`, and `realized`?
+16. Which edge cases of `Position.update` would you test? Think about zero, flips, shorts, and adding to a short.
+17. Should prices use `float`? What would you use in production, and why?
+
+## Tests
+
+18. Which existing tests could never fail? Explain why each one passes.
+19. How do you show that a new test really detects a bug?
+20. Would property-based testing help here? What property would you check?
+21. What would you add to CI so that weak tests like these get caught?
+
+## Wrap-up
+
+22. If you could ship only one fix today, which one would you pick, and why?
+23. What would you add so Ops can answer this kind of ticket without engineering?
+24. How did you check the AI's suggestions? Where was it wrong, or where did it not help?
+25. In production, how would you find positions that were already corrupted by these bugs?