explain: route metadata-blob prefetch through refs.Primary
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loadV1MetadataRootTree is renamed to loadPrimaryMetadataRootTree and takes CommittedRefs. The origin remote-tracking fallback is gated on Read == Primary && PrimaryFetchableFromOrigin so the local-only mirror ref skips it.
Entire-Checkpoint: 9b09e42a8892
Checkpoints
Checkpoint 1