[Twinkle-3000]

Summary

• Add pytest coverage for scene data, camera transforms, mesh preparation, picking, capture, incremental refresh, and Qt interactions.
• Add a GitHub Actions workflow to run the test suite on pushes, pull requests, and manual dispatches.
• Add project guidelines for architecture, coding, and testing.

Validation

• CI installs dependencies with uv sync --locked --extra test and runs uv run pytest on Python 3.11 with Qt in offscreen mode.

The workflow is in tests.yml, and the new tests are in tests/.