fix: P2P: block merkle roots not validated before inserting into archive DB

Bug: s2-r22 (port)
Finding: https://giaki3003.tech/#/findings/20260603-2200-thunder-peer-body-poison-archive-and-net-task-halt
Severity: Medium

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: giaki3003