Publish Reader and Writer documentation, apps listings and videos

[callumalpass]

Summary

• Refresh and publish the nine Reader guide drafts against committed app behaviour: production URL, Chrome Web Store install, capture controls, imports, saving, collection links, and retired offline copies.
• Add six Writer guides covering setup, writing/review, citations, chapters, layouts/export, and saving/data.
• List Reader and Writer on the apps page with application, documentation, preview/demo and extension links.
• Integrate five sample-data videos supplied by the recording agent. Serve MP4s and compressed posters locally with native controls, no autoplay, preload=none, downloads and text equivalents. Authenticated GitHub attachment URLs are not used for public playback.
• Reuse the draft multi-section DocsLayout while retaining SDK navigation. Leave unrelated Editor/MCP drafts out.
• Add documentation/media regression tests to CI and production builds.

Validation

• Documentation tests and existing specification/homepage/deployment/release-update tests pass.
• Astro check, static build and Connect release verification (beta.121) pass.
• Specification import and all 48 generated pages pass local-link checks.
• Headless Chromium: 17 routes at 390px and 1280px, no overflow or page errors, no automatic MP4 requests; all five MP4s decode/play and their text descriptions open.
• git diff --check passes.

Deployment

Merge triggers the existing production GitHub Pages workflow. No Cloudflare production deployment or repository visibility changes. Existing dirty canonical checkouts and unrelated drafts are untouched.