fix: avoid import_batch shared-dep panic

[lodystage]

Summary

• skip duplicate VV evaluation when shared deps queue the same DAG node twice during ensure_vv_for
• add an import_batch regression test built from public import_json_updates input so causally valid redundant deps no longer panic

Testing

• cargo test -p loro-internal --test import_batch_shared_dep
• cargo test -p loro-internal import_batch