Add smoke coverage for the primary workflow

[ChrisFox-Eth]

Why

The repository now has a public build workflow. A focused smoke test would make the project easier to maintain and safer for outside contributors to change.

Scope

• Add one small test or validation script for the primary user flow.
• Prefer fixture-driven checks over brittle UI snapshots.
• Document the command in README.md if a new script is added.

Acceptance criteria

• The smoke path fails on a meaningful regression and passes on the current sample data.
• The test uses only synthetic fixture data.
• GitHub Actions runs the check through the existing build workflow.