Add PSR-6 cache adapter

[vanderlee]

What changed

• Add a Vanderlee\Syllable\Cache\Psr6 adapter for PSR-6 cache pools.
• Preserve the existing Cache strategy and Syllable::setCache() API.
• Support psr/cache versions 1, 2, and 3 so Symfony Cache and other PSR-6 implementations can be used.
• Add adapter lifecycle and cache-miss tests.
• Document Symfony Cache usage without making Symfony a runtime dependency.
• Allow PHPUnit 9.6 on modern PHP so PSR-6 v3 interfaces can be mocked, while retaining the older PHPUnit ranges for PHP 7.1.

Why

Issue #24 proposed interchangeable internal components without breaking legacy integrations. Namespacing and the strategy interfaces are already present; this PR completes the concrete PSR-6 caching part as an additive adapter and leaves the optimized hyphenation algorithm untouched.

Compatibility

Existing file, JSON, serialized, and custom Cache implementations continue to work unchanged. Applications opt into PSR-6 explicitly by passing the adapter to the existing setter.

Validation

• composer validate --no-check-publish --no-check-lock
• PHPUnit functional suite: 48 tests, 121 assertions; 2 existing warnings and 2 existing incomplete tests
• PHP syntax checks for the adapter and its tests

Closes #24.