[beinan]

Summary

• discover top-level *.rollout.lance datasets through Lance object-store APIs during master startup
• exclude _registry and _stats, then batch-insert only missing registry entries in one append
• add idempotency, filtering, URI joining, and startup migration regressions

Relationship

• merge after the registry reload PR so master also observes registry commits made after startup by workers
• this PR is independently based on main and addresses the legacy-store migration gap

Testing

• cargo fmt --all -- --check
• cargo test -p lance-context-core -p lance-context-master
• cargo clippy --workspace --all-targets -- -D warnings