Oci Module Caching

oci - Content-addressable caching for OCI module sources

oci:// module sources now integrate with Content Addressable Storage. When the oci experiment is enabled, downloads are cached by their manifest digest, so a repeated fetch of the same tag or digest is served from the local store instead of re-downloaded from the registry.

Mutable tags stay correct: every fetch re-resolves the tag to its current manifest digest at download time, so re-pushing a module under the same tag invalidates the cache and pulls the new content rather than serving a stale copy. A digest-pinned source (?digest=sha256:...) skips registry resolution and keys the cache directly.