fix(docs): repair migration link and mermaid label clipping

[robertsLando]

Summary

• Broken link on landing page — docs-site/index.md: the [migration guide](/guide/migration) link inside <div class="fork-banner"> was rendered as literal text because raw HTML blocks don't parse inline markdown without blank-line separation. The div now wraps blank-line-separated markdown content so the link resolves to /pkg/guide/migration.
• Mermaid label clipping — in every multi-line node label, the last line was cut off below the node rect (e.g. User code / fs.readFileSync / /snapshot/app/x.js, Executable with / custom VFS, Traditional / walker + bytecode). Mermaid renders label text through a markdown parser that wraps lines in <p> tags; the browser's default <p> margin inflated the rendered label height beyond the foreignObject height mermaid had pre-measured. Added CSS to zero <p> margins inside .nodeLabel / .edgeLabel / .cluster-label / .label and normalized line-height.
• Mermaid font alignment — .vitepress/config.ts: set themeVariables.fontSize: '15px' and flowchart.padding: 12 so mermaid's layout math matches the 15px font-size applied by custom.css.

Test plan

• npx vitepress build succeeds with no dead-link errors
• Verified in preview that the landing page renders the migration link as a proper anchor
• Verified that all mermaid diagrams on /pkg/architecture render every label line inside its node box with no bottom clipping — both 2-line and 3-line labels
• Pre-commit hook (lint-staged + prettier) passes

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