Replace magic numbers with named constants across all language implementations

[Copilot]

This PR addresses code maintainability by replacing magic numbers with appropriately named constants across all language implementations in the refactoring kata.

Changes Made

Magic Numbers Replaced

Formatting Constants:

• 2 → PRICE_DECIMAL_PLACES for currency formatting precision
• 0 → WHOLE_QUANTITY_DECIMAL_PLACES for integer quantity display
• 3 → FRACTIONAL_QUANTITY_DECIMAL_PLACES for fractional quantity display

Business Logic Constants:

• 2 → TWO_FOR_OFFER_QUANTITY for "two for amount" special offers
• 3 → THREE_FOR_TWO_QUANTITY for "three for two" special offers
• 5 → FIVE_FOR_OFFER_QUANTITY for "five for amount" special offers
• 100.0 → PERCENTAGE_DIVISOR for percentage discount calculations

Layout Constants:

• 40 → DEFAULT_COLUMN_WIDTH for receipt formatting
• 1 → DEFAULT_ITEM_QUANTITY for standard item quantities
• 3 → DISCOUNT_SPACING_OFFSET for receipt layout spacing

Implementation Details

Each language follows its own conventions for constants:

C++: Used constexpr constants in the private section for compile-time evaluation
C: Used #define macros in the header file following C conventions
TypeScript: Used static readonly properties for type-safe class constants
Ruby: Used class constants with ALL_CAPS naming convention
Elixir: Used module attributes (@constant) for immutable compile-time constants

Benefits

• Improved Readability: Code intent is now self-documenting through meaningful constant names
• Easier Maintenance: Changes to formatting or business rules only require updating one location
• Reduced Errors: Eliminates the risk of using incorrect magic numbers in calculations
• Better Documentation: Constants serve as inline documentation for business rules

Testing

All existing tests continue to pass, confirming that the refactoring maintains backward compatibility:

• C++ (GTest): ✅
• C (Catch2): ✅
• TypeScript (Mocha): ✅
• Ruby: ✅
• Elixir: ✅ (syntax validated)

Fixes #3.

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