docs: explain external I/O fencing architecture

[sqlrush]

Summary

• add a Chinese, diagram-rich external I/O fencing architecture guide
• distinguish Oracle-verified behavior from PGRAC's provider-neutral adaptation
• document terminal readback, proof binding, durable journal, recovery admission, rejoin, fail-closed behavior, deployment certification, and diagnosis
• state the current public-main boundary honestly: no certified production provider is registered, so production activation remains closed
• link the guide from the architecture overview

Verification

• 678 lines, 12 Mermaid diagrams
• Markdown fences and supported Mermaid headers checked
• relative public source anchors checked
• private Spec/AD/roadmap terminology scan clean
• remote branch contents and overview links verified