test: integration test for capture → compare pipeline

[tom-bot-for-rana]

Problem

capture-routes.test.js mocks Playwright entirely and only tests the capture module in isolation. There are zero tests that run the full capture → compare → stage pipeline, even with a tiny synthetic fixture. Interface mismatches between modules (e.g. a field rename in the results JSON) would not be caught by unit tests.

Proposed fix

Add one integration test that exercises the full data flow using pre-built PNG fixtures (no live browser needed):

1. Write a minimal snapdrift.json config with 1–2 routes
2. Place small pre-built PNG files in the expected screenshots/ location (bypassing Playwright entirely)
3. Run generateDriftReport() directly against the fixture screenshots
4. Assert that the output visual-diff-summary.json has the correct shape and values
5. Optionally stage the bundle and verify the artifact structure

This tests the compare-results → stage-artifacts interface without needing a running browser.

Scope

• New tests/integration/capture-compare-pipeline.test.js
• Small PNG fixture files in tests/fixtures/

Effort

Medium — requires PNG fixture generation and careful test setup, but no new lib code needed.

[tom-bot-for-rana]

PR opened: #43