security: threat model + adversarial test plan (DoS, eclipse, sybil, partitions)

## Goal
Document and test against realistic adversarial scenarios for mainnet readiness.

## Scope
- Threat model doc (attack surfaces + mitigations)
- Test plan covering eclipse/sybil/DoS/partitions/replay
- Add adversarial integration tests where feasible

## Acceptance criteria
- Threat model reviewed and tracked
- CI contains at least a minimal adversarial suite