fix: resolve MDX values in Markdown exports

[abernier]

Summary

Copy Page, View as Markdown, and llms-full.txt currently expose raw MDX expressions instead of the values shown in the documentation. Use one shared renderer to evaluate the consumer's scope and components, then serialize the rendered document as Markdown. For example, a spacing token expression now exports 1rem.

Preserve headings, lists, tables, links, footnotes, task lists, and literal code, including fence metadata. Provide static representations of built-in components and optional markdownComponents overrides for interactive custom components. Export all tab panels and package-manager commands; opaque client components receive an explicit fallback. Source MDX files remain unchanged.

Includes documentation and a patch changeset.

Validation

• pnpm run lgtm: passed before commit — 672 tests passed, 4 skipped; existing VersionSwitcher lint warning only.
• PORT=3198 pnpm run LGTM: passed — 124 browser tests, including clipboard equality with the Markdown endpoint and matching content in llms-full.txt.
• Production static export against design-system commit 8562025: all 11 documents exported, all 31 literal code blocks preserved, page Markdown matched llms-full.txt, and source SHA-256 hashes remained unchanged.
• Real design-system Copy Page clipboard matched its Markdown endpoint in a headless browser.