feat(core): disk cache storage for generated variants

[dangreen]

Adds SrcSetCacheStorage - a disk cache of generated image variants.

• memo skips repeated generation: variants are stored on disk with a manifest and read back on repeated generation with the same source, options and variant
• Cache addresses are derived from the inputs before generating: manifests are keyed by a sha256 of the source contents, path and generation options; files are stored under the plain variant name for direct serving
• Manifest stores a contents hash: files overwritten by a colliding name are a miss with regeneration
• Atomic writes (tmp + rename), flat-name path guard, sharp versions and a cache schema version in the key
• SrcSetGenerator accepts the storage via the cache option; variant naming is unified in resolveVariant, shared by the generator and the storage

🤖 Generated with Claude Code