Add comprehensive mainnet safety implementation plan

Created detailed 6-week plan to implement 5 critical safety mechanisms
to prevent state rollbacks before mainnet launch.

Safety mechanisms:
1. State monotonicity check - Prevents loading old state files
2. Peer state verification - Cross-checks state with network
3. Immutable state archives - Prevents accidental overwrites
4. Deployment safety checks - Pre-deploy validation scripts
5. Emergency circuit breaker - Halts node on rollback detection

Each mechanism includes:
- Detailed design and code examples
- Integration points
- Testing requirements
- Deliverables
- Timeline

Timeline: 6 weeks total
- Week 1: State monotonicity check (CRITICAL)
- Week 2: Peer state verification
- Week 3: Immutable state archives
- Week 4: Deployment safety checks
- Week 5: Emergency circuit breaker
- Week 6: Testing & documentation

Success criteria defined with must-pass tests before mainnet.
Risk assessment shows catastrophic impact if not implemented.

This plan addresses the testnet rollback event (round 4047 → 50)
and ensures such events cannot occur on mainnet.

Author: UltraDAGcom