Rewrite backfill plan with concrete step-by-step procedure

Key decisions documented:
- Direction: newest→oldest (HEAD is golden reference, fail-fast)
- Versions: only the 7 versions actually shipped via duckdb-vortex bumps
- Inclusion criteria: builds + SQL runs + file non-empty + round-trips
- Anti-drift: frozen SQL (never modified per version), self-verify row
  counts, manifest commit hashes, golden CSV diffs, SHA-256 checksums,
  SQLLogicTest assertions — six layers of verification
- Subagent template with CRITICAL RULES section

https://claude.ai/code/session_01S2WAcroYU2s7u3gJzGwCCe

Author: claude