fix(core): avoid full-scan rollout pagination refetch

[beinan]

Summary

• collapse list_filtered_source from the two-phase page-id plus id IN (...) refetch into one projected, filtered, bounded scan
• use the direct Lance dataset scanner for the master-default fragments source so limit/offset push down without an LSM-wide sort
• retain one-pass LSM scans for wal and all so cross-generation primary-key deduplication remains intact
• add multi-fragment wide-row pagination coverage and update the scale benchmark for direct fragment scans

Testing

• cargo test -p lance-context-core rollout_store::tests::
• ./.codex/skills/ci-pr-helper/scripts/run_ci_checks.sh