Test PR: Demonstrate CI/CD workflow

[stphung]

This is a test PR to demonstrate that CI/CD runs on pull requests.

Purpose

• Show that all 4 platforms (Linux, Windows, Web, Android) build on PRs
• Demonstrate test execution on PRs
• Verify that versioning and releases do NOT run on PRs

Changes

• Minor README update with test marker

This PR will be closed after CI/CD demonstration.

[stphung]

Closing test PR. Successfully demonstrated that CI/CD triggers on pull requests, though this run failed due to Docker Hub authentication issues in GitHub Actions infrastructure (not a project configuration issue).