Exercise 3: A Second New Shipping Type — Possible Solution
====================================================================

THE NEW CLASS
------------------------------
    class EconomyShipping(ShippingStrategy):
        def calculate(self, weight): return weight * 0.3 + 2

Added alongside this chapter's own InternationalShipping, bringing the
hierarchy to five classes total: StandardShipping, ExpressShipping,
OvernightShipping, InternationalShipping, and now EconomyShipping.

RESULTS
------------------------------
    All 4 pre-existing classes verified byte-identical after adding
    EconomyShipping:
      StandardShipping unchanged: True
      ExpressShipping unchanged: True
      OvernightShipping unchanged: True
      InternationalShipping unchanged: True

    EconomyShipping.calculate(10) = 5.0, hand calculation = 5.0, match = True

Every class that existed before this exercise - including
InternationalShipping, itself only added in this chapter's own main
text - stays byte-identical after EconomyShipping is added. The new
class's own cost calculation matches a hand-computed check exactly.

WHY THIS WORKS AS AN ANSWER
------------------------------
This confirms the chapter's own central claim doesn't only hold for
the first extension beyond the original three classes - it holds for
every extension after that, indefinitely. InternationalShipping was
itself proof that adding a new type doesn't disturb the three original
classes; this exercise proves that InternationalShipping, once added,
becomes just as stable as the three it joined. Each new shipping type
added this way permanently strengthens the guarantee rather than being
a one-time result that might not generalize further.