Rewrite backfill plan: pure Rust, forward direction, no DuckDB needed

Key changes from previous plan:
- Use vortex Rust crate directly, not DuckDB SQL
- Go forward (oldest→newest) so each version validates against previous
- Use existing java/testfiles/src/main.rs as template (already compiles
  at each tag, same 10-row dataset)
- 4 layers of anti-drift: frozen data, self-verify read-back,
  forward-chain diff, final golden comparison
- Document 4 API eras (0.40–0.53, 0.54–0.55, 0.56–0.62, 0.63+)
- Subagent only changes the output path, not the data

https://claude.ai/code/session_01S2WAcroYU2s7u3gJzGwCCe

Author: claude