Add typing-aware keyboard shortcut example

[lukeslp]

Summary

Add a dependency-free keyboard shortcut example that keeps native buttons as the primary controls and provides an explicit off switch. The guard respects editing, composition, repeat events, application ownership, shadow DOM and unavailable actions.

The example includes a guide and executable browser checks. Existing packages, versions and plugin installations are unchanged.

Validation

• 34 Chromium checks passed, including real typing, dialog focus restoration and 390px layout overflow. Desktop and mobile screenshots were inspected.
• Missing-browser failure exits promptly and closes the test server.
• Repository tests, lint, formatting, spelling and package-content checks passed.
• Screen-reader output, speech input and other browsers remain untested; no overall conformance claim is made.