ci(docs): build the live demo from the released vsix

[lukecotter]

📝 PR Overview

The publish workflow built the live demo from a second checkout of the latest stable release. It installed that checkout's dependencies, then rebuilt the viewer with NODE_ENV=production pnpm run build:dev, because old releases have no build:prod. The demo then ran a rebuild of the release, not the release itself.

This PR downloads the released vsix from Open VSX and builds the demo from it. The demo now runs the same bytes that users install.

🛠️ Changes made

• Publish workflow:
  • It downloads FinancialForce/lana at the release tag from Open VSX. It asks by version, because Open VSX's latest can be a pre-release.
  • It unzips the vsix to $RUNNER_TEMP/release and renames extension to lana, so build-demo.mjs reads it with no code change.
  • The download runs in parallel with the dependency install.
  • Removed: the release checkout, the release install, the release lockfile in the cache key, and the build:dev workaround.
  • Test Demo Host runs without DEMO_VIEWER_ROOT. The test scans log-viewer/src, and the vsix does not have it. So publish now checks main's host against main's viewer, not against the released viewer.
• build-demo.mjs and lana-docs/README.md: DEMO_VIEWER_ROOT can now be any folder with a built lana/out and lana/package.json, such as an unzipped vsix.

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

🔗 Related Issues

Follows #1147.

✅ Tests added?

• 👍 yes

• 🙅 no, not needed

• 🙋 no, I need help

• The download step, run locally for 1.22.1, gives lana/out and lana/package.json.

• The site builds from it, and pnpm --filter docs-site test passes.

• The demo's bundle.js is byte-identical to the one in the 1.22.1 vsix.

📚 Docs updated?

• 🔖 README.md
• 🔖 CHANGELOG.md
• 📖 help site
• 🧪 Marked any pre-release-only features (README 🧪 badge — see RELEASING.md)
• 🙅 not needed

Anything else we need to know? [optional]

• The publish workflow runs only after a release or by hand, so CI does not run this change. Run the workflow by hand after merge to check it.
• Publish now needs Open VSX. CI e2e already needs it for Salesforce Services.