feat: resolve a BackedEnum case by name (fromName/tryFromName)

[k2gl]

Summary

Native BackedEnum only offers from()/tryFrom(), which resolve a case by its backing value. There is no built-in way to resolve a case by its name (the case identifier), so callers hand-roll a cases() loop whenever they hold a name string from config, serialized data, a CLI argument or a route parameter.

This PR adds the symmetric pair, mirroring the native API:

CardSuit::fromName('SPADES');    // CardSuit::SPADES
CardSuit::tryFromName('SPADES'); // CardSuit::SPADES
CardSuit::tryFromName('joker');  // null
CardSuit::fromName('joker');     // throws \ValueError

• fromName(string $name): static — throws \ValueError on an unknown name.
• tryFromName(string $name): ?static — returns null on an unknown name.
• Name matching is case-sensitive and does not match backing values.

Both are added to ExtendedBackedEnum (trait) and ExtendedBackedEnumInterface. Consumers using the trait get the implementation for free.

Also in this PR

• README fix: the usage example imported non-existent namespaces (K2gl\Enum\src\…, K2gl\Enum\Types\…); corrected to the real K2gl\Enum\… paths so the snippet actually compiles.
• PHPStan level 6 → 9, aligning the package with the rest of the collection. No source changes were required to pass.

Tests

New FromNameTest / TryFromNameTest cover valid names, unknown names, case-sensitivity and the "backing value is not a name" case for both string- and int-backed enums. Full gate green: PHPUnit (57 tests), PHPStan level 9, Pint.