refactor: use PSR-16 exclusively for all caching in JsonLoader

Replace custom in-memory array cache with PSR-16 ArrayAdapter as default.
This simplifies the implementation and enables multi-tier caching via
Symfony's ChainAdapter (memory + filesystem).

- Add symfony/cache dependency for ArrayAdapter default
- Default to ArrayAdapter when no cache provided (in-memory deduplication)
- Remove custom $memoryCache array in favor of PSR-16
- Update documentation with multi-tier caching examples (ChainAdapter)
- Update tests to reflect PSR-16-only architecture

Users can now configure any caching strategy:
- Default: ArrayAdapter (in-memory, request deduplication)
- Persistent: FilesystemAdapter, RedisAdapter
- Multi-tier: ChainAdapter([ArrayAdapter, FilesystemAdapter])

Author: CybotTM