Mermaid support

[Sam Clark (28pins)]

Summary

Added optional (default on) mermaid support; uses beautiful-mermaid-swift. MermaidConfig lets user change settings or disable.

Validation

Added tests and checked kitchen sink demo

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

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