Exercise 2: The Bulk-Discount Boundary — Possible Solution
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DISCOVERY
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    legacy_calculate_shipping(50, 'A', False) = 25.0     (boundary itself)
    legacy_calculate_shipping(50.01, 'A', False) = 22.5  (just above)

    Hand check: 50 * 0.5 = 25.0, no bulk discount applied - confirms
    the condition weight > 50 is strictly exclusive of 50 itself.

THE CHARACTERIZATION TEST
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    def test_char_bulk_discount_boundary():
        assert legacy_calculate_shipping(50, 'A', False) == 25.0  # NOT discounted

RESULT
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    Characterization test: PASS

WHY THIS WORKS AS AN ANSWER
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This captures a specific, easy-to-get-wrong detail: whether the bulk
discount's own threshold is inclusive or exclusive of 50 itself. None
of the chapter's own original 6 tests happened to probe this exact
boundary - the closest was weight=60, comfortably past it. Discovering
and capturing the boundary value specifically matters because
off-by-one errors are exactly the kind of change a well-meaning
refactor might introduce without anyone noticing, and exactly the kind
of change this exercise's own test is positioned to catch that the
chapter's original suite could not.