Release 3.0.0: CountryCode enum migration

[3.0.0] - 2025-10-02

Added

• CountryCode enum replacing raw string codes.
• FormatterRegistry redesigned to be immutable (register() returns a new instance).
• FormatterRegistry::all() method for inspecting registered formatters.
• Full PHPUnit test coverage for registry immutability and enum integration.

Changed

• BC BREAK: All methods now expect CountryCode instead of string country codes.
• BC BREAK: FormatterRegistry no longer has static state – must be instantiated and passed into PostcodeFormatter.
• PostcodeFormatter now requires a FormatterRegistry instance in its constructor.

Fixed

• Clearer distinction between UnknownCountryException and UnsupportedCountryException.
• Improved error messages with translation keys (error.invalid_postcode, error.unknown_country, error.unsupported_country).

Tests

• ✅ Full PHPUnit coverage (100%).
• ✅ PHPStan Level 10 + Strict Rules passing with 0 errors.