- Fix cargo bouncing in relay chains: piece-to-piece transfers now prefer
receivers farther from production (forward flow) instead of closer (backward)
- Level 2 stock corrected: 4R+1B → 6R+1B to match required solution
- Level 3 simplified to single cargo type (6R+1K) — dual-cargo on 6x6
requires engine support for cargo-type filtering (Phase 2)
- Add PLAN.md with prototype roadmap (phases 2-6)
- 57 tests passing
