docs: accuracy review of every docs page against 3.0 (props, samples, nav gaps) + guard tests #123

Deep review of site/src/content/docs against the 3.0.0 package: wrong/outdated props, defaults, imports and samples; pages missing for exported components or the nav; getting-started must match bunx @codeatlasdev/ui init.

Acceptance: list of inaccuracies fixed in the PR; guard test failing when a documented prop is absent from the manifest; MDX code blocks type-checked; validate + docs e2e pass.