Add backward compatibility test data backfill plan

Detailed plan for generating .vortex fixture files across all historical
vortex releases (0.26.0 through 0.63.0) to enable backward compatibility
testing. Includes fixture definitions, directory layout, per-version
generation procedure, anti-drift checksums, and subagent task template.

https://claude.ai/code/session_01S2WAcroYU2s7u3gJzGwCCe

Author: claude