feat(labs): add Material 3 navigation rail

[Ankith-B]

Summary

• add experimental md-navigation-rail built on the existing navigation bar/tab primitives
• support vertical layout and Arrow Up/Down keyboard navigation while preserving Home/End and activation behavior
• add demo, harness, focused coverage, and component documentation

Evidence and scope

This addresses the open navigation rail request in #5608 and the navigation component gap documented in the roadmap. The implementation is intentionally in labs/ because the repository is in maintenance mode and the existing navigation bar/tab components are experimental.

Validation

• npm run build:ts -- --no-pretty
• npm run build:sass
• npx wtr --files labs/navigationbar/md-navigation-bar_test.js --config web-test-runner.config.js --concurrency 1 (38 passed)

The initial standalone rail fixture timed out because its environment lifecycle interacted poorly with the test harness. The coverage now lives in the existing navigation bar suite and completes successfully. Acceptance is uncertain: the roadmap says no new components are currently planned, so maintainer direction and design review may be required.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[Ankith-B]

@googlebot I signed it!

[Ankith-B]

@googlebot I signed it!

[asyncliz]

Thank you for the contribution! Unfortunately right now we're not accepting new component PRs due to the overhead it takes to review and import them.