- Add mainnet+testnet checkpoints at blocks 2190000, 2200000, 2205000
- Bump MAX_ORPHAN_BLOCKS from 750 to 2000 (prevents fork deadlocks)
- Add MODERNIZATION_ROADMAP.md with prioritized improvement plan
These changes prevent the exact fork deadlock that happened during
the Apr 17-19 incident: post-IBD orphan limit of 750 was too low,
causing nodes to deadlock when divergent blocks arrived.
Krystie