Exercise 3: A Second Customer's Redemption — Possible Solution
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SETUP
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    Customer 1 (GOLD, from the chapter's own final integration check):
      40 points balance, redeems all 40

    Customer 2 (PLATINUM, new for this exercise):
      25 points balance, redeems all 25

RESULTS
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    Customer 1 (GOLD, 40 pts redeemed): $4.00
    Customer 2 (PLATINUM, 25 pts redeemed): $2.50

    Customer 1 recheck (unaffected by Customer 2's own call): $4.00,
      matches original: True

    Customer 2 negative points correctly rejected:
      "cannot redeem a negative number of points"

    Expected: Customer 1 = $4.00, Customer 2 = $2.50
    Match: True

WHY THIS WORKS AS AN ANSWER
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This confirms calculate_redemption_discount() - the single shared
function Step 7's own code review required, replacing the two
duplicated copies - genuinely has no hidden shared state between
calls: Customer 1's own result stays exactly $4.00 whether checked
before or after Customer 2's own independent redemption, and the
negative-points rejection Step 3's standup blocker required works
identically for a second, different customer without needing separate
handling. This is the same guarantee Clean Code, SOLID & Refactoring's
own capstone and Software Testing Strategy's own capstone both verified
for their respective shared functions - a properly extracted, pure
function processes each call independently, with no risk of one
customer's own transaction leaking into another's.