docs: fix broken mermaid diagram in design.md

[google-labs-jules]

This PR fixes a Mermaid diagram syntax error in docs/design.md. The issue was caused by unquoted parentheses in node labels (e.g., SandboxMgr[Jp2kSandbox (Singleton)]), which confused the parser.

Changes:

• Enclosed node labels in double quotes in the Architecture Overview diagram to ensure correct parsing of parentheses and slashes.

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PR created automatically by Jules for task 4932372999407852670 started by @keiji

[google-labs-jules]

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