Replace magic numbers and strings with named constants across all implementations

[Copilot]

Issue #23 requested elimination of magic numbers and strings from the codebase.

Changes

Replaced hardcoded values with named constants across 15 language implementations (Python, Java, TypeScript, C#, Go, Ruby, Swift, Kotlin, C++, C, PHP, Elixir, Common Lisp):

Numbers → Constants:

• 1.0 → DEFAULT_ITEM_QUANTITY
• 2, 3, 5 → TWO_FOR_AMOUNT_QUANTITY, THREE_FOR_TWO_QUANTITY, FIVE_FOR_AMOUNT_QUANTITY
• 100.0 → PERCENTAGE_DIVISOR
• 40 → DEFAULT_COLUMNS (receipt printer)

Strings → Constants:

• "3 for 2" → THREE_FOR_TWO_DESCRIPTION
• "2 for " → TWO_FOR_PREFIX
• " for " → FOR_SEPARATOR
• "% off" → PERCENT_OFF_SUFFIX
• "Total: " → TOTAL_LABEL (receipt printer)

Example (Python)

Before:

def add_item(self, product):
    self.add_item_quantity(product, 1.0)

# ...
if quantity_as_int >= 2:
    discount = Discount(p, "2 for " + str(offer.argument), -discount_n)

After:

DEFAULT_ITEM_QUANTITY = 1.0
TWO_FOR_AMOUNT_QUANTITY = 2
TWO_FOR_PREFIX = "2 for "

def add_item(self, product):
    self.add_item_quantity(product, DEFAULT_ITEM_QUANTITY)

# ...
if quantity_as_int >= TWO_FOR_AMOUNT_QUANTITY:
    discount = Discount(p, TWO_FOR_PREFIX + str(offer.argument), -discount_n)

No behavioral changes. All existing tests pass.

Original prompt

Create a solution for issue #23

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