fix: resolve MDX values in Markdown exports

[abernier]

Summary

Markdown exports currently expose unresolved MDX expressions such as {spacing['4'].value}. Resolve expressions and configured components through the Next application so Copy Page, /<page>.md, and llms-full.txt return the same rendered Markdown, including values inside tables, links, and commands.

Preserve literal code and fence metadata, expose static representations of built-in components, and support markdownComponents overrides for interactive components. Sources remain unchanged; unresolved expressions fail with document context. Markdown route handlers use the same Next builder as the website and HTML fragments; React stream serialization happens inside those handlers.

Stack

• Base: refactor: share Next.js rendering for fragments and websites #717 (codex/next-fragment-rendering), which targets main.
• Replaces closed fix: resolve MDX values in Markdown exports #715 and preserves its Markdown work, with additional image-resolution regression coverage.
• Merge refactor: share Next.js rendering for fragments and websites #717 first, then retarget this PR to main.

Pre-merge review

Generated Entries links now include BASE_PATH, matching authored links and website navigation. Regression tests cover a configured, empty, and absent prefix, preserve group filtering, and check that authored links are not prefixed twice.

Validation

• pnpm run LGTM on 343f8cfc: typecheck, lint/format, registry validation, 681 passing unit/integration tests (4 skipped), and all 120 Playwright tests passed.
• Coverage includes scope evaluation, async components, Markdown overrides, literal code and metadata, image URLs, and equivalent page/aggregate exports without source changes.
• Full documentation Next production export: 50 pages generated. Targeted Copy Page/View as Markdown Playwright test: 2 passed, desktop and mobile viewports, headless Chrome for Testing.
• An additional fresh CLI consumer-website smoke build could not complete because Next failed to download Inter and Inconsolata from Google Fonts. The full repository production export and browser checks above completed successfully.
• Existing lint warning in VersionSwitcher.tsx remains; no lint errors.