Add a dedicated WebGPU example for bracket-terminal

[mors119]

Summary

Add at least one dedicated WebGPU example to bracket-terminal.

Problem to solve

bracket-terminal currently has examples for other rendering paths, but it does not appear to have a dedicated example for the WebGPU backend.

This makes it harder to:

• verify that the WebGPU path works
• test WebGPU-specific behavior during maintenance
• use an example as a reference when adding new WebGPU features

Proposed solution

Add one or more WebGPU-focused examples that can be run with the WebGPU feature enabled.

A minimal example would be enough to start, as long as it helps confirm that the WebGPU backend is working and provides a reference for future development.

Alternatives considered

• Rely only on existing non-WebGPU examples
• Delay adding examples until more WebGPU work is done

Additional context

A dedicated WebGPU example would be useful both for regression checking and for contributors working on backend-specific changes.