Task 1: Stabilize Mermaid diagram rendering across documentation vault

Status: done
Priority: medium
Source PRD: .taskmaster/docs/prd-mermaid-diagram-rendering-stabilization.md

Correct Mermaid syntax/rendering issues across the documentation vault and validate all Mermaid blocks.

The source PRD records that all targeted task files pass @mermaid-js/mermaid-cli parsing and the full vault sweep passes.