Render Mermaid diagrams in the viewer

[davidmoshal]

Fixes #14

Summary

• Render Mermaid fences in initial and dynamically stacked viewer panels.
• Lazy-load pinned Mermaid 11.16.0 from embedded same-origin assets in strict mode.
• Match viewer theme tokens and add contextual accessible names.
• Preserve source plus inline errors for invalid diagrams.
• Include the runtime and assets in default interactive HTML exports.
• Add Node adapter tests, Go integration/export tests, documentation, notices, and licenses.

Root cause

Markdown recognized the fence, but the viewer emitted only a generic code block; no Mermaid runtime or attached-panel enhancement existed. Real-browser QA also caught that detached dynamic panels cannot safely render layout-dependent diagrams, so enhancement now runs after panel attachment.

Validation

• pnpm test:cli
• go test -race -shuffle=on -count=1 ./packages/cli/...
• go vet ./packages/cli/...
• source-built openknowledge validate Wiki
• source-built Chrome QA: initial render, invalid fallback, later-valid isolation, stacked sequence diagram, theme switch, and mobile scroller
• interactive HTML export Chrome QA
• five-lane exact-SHA review plus independent debugging runtime audit

The vendored Mermaid bundle is approximately 3.4 MB but is fetched only when a Mermaid fence exists. Its version, MIT license, and SHA-256 are recorded in the third-party notices.