First standalone release. Provides:
- StorageInterface + StorageManager
- Adapters: LocalFilesystem, S3, CloudflareImages, InMemoryStorage
- Variant pipeline via ImageResizer + VariantRegistry
- Array-driven StorageConfig (no framework-specific config dep)

Adapter SDKs (league/flysystem-aws-s3-v3, cloudflare/sdk,
gumlet/php-image-resize) are suggested rather than required so
consumers only pull in what they use.

Pre-1.0: API may break between minor versions until validated by a
second consumer.