Mermaid lazy render when tab becomes visible

[yoshibase]

Summary

Fixes #8357

Fix

Defer Mermaid rendering until the diagram container is visible (IntersectionObserver). Hidden tab panels no longer render incomplete SVG arrow markers.

Also sanitizes React useId() output for Mermaid marker IDs.

Test plan

• Manual: dogfooding page with Mermaid in non-lazy tabs
• Verify arrows render in second tab after switching

Submit

make submit CONTRIB=contrib-facebook-docusaurus/pr-8357-mermaid-tabs

Test plan

• Local tests pass (see README in staging folder)

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